When Derek Smith agreed to let a Channel Four film crew into King's College Hospital to make the documentary Operation Hospital six years ago, he did not think it would be remembered mainly for its anti-doctor stance and for his choice of car. 'Things do not always go the way you plan,' he says.

The chief executive of King's Healthcare trust has since traded in the blue sports car that so inflamed local unions for a Ford Fiesta. The thoughts of those doctors whose egos were bruised by the series after being depicted as arrogant and uncaring are not recorded.

'We wanted to demonstrate to people we were doing a lot to improve the services and performance of the hospitals, and we thought this would be a good vehicle for doing it. But the angles the journalist chose to follow were not those discussed at the outset,' he says.

Despite serious reservations - 'when it came to presentation of finance, they got it wrong' - he also believes that the series, which attracted 1.5 million viewers, was ultimately beneficial.

'I think in the local community we did actually manage to sell the idea that things were changing,' he says.

Mr Smith advises anyone considering whether or not to allow the programme- makers in to agree the likely 'line' of the programme before filming starts.

They should also ensure a long lead-in time to allow hospital and staff to prepare for the film crew's arrival, and they should arrange for a pre-transmission screening to correct factual errors. 'Check whether the risks outweigh the benefits. It is a difficult judgement to make but a crucial one,' he says.

The dilemma of whether or not to co-operate with programme-makers seems to be increasingly common for managers. Some believe that as the median age of the population increases, so does the interest in health.

'Hardly a week goes by when we are not asked if we would let television crews in for one reason or another,' says Ros Cliffe, director of public affairs and general fundraising at Great Ormond Street Hospital for Children trust - in television terms perhaps the most magnetic location of all.

Even the less obviously 'glamorous' hospitals have many requests. Brighton Healthcare trust gets four proposals in a busy week from film crews. 'Word quickly gets around that it's a great place to film at,' says Elizabeth Bell, press and public relations manager.

So how do you decide which television programmes to allow in and which to exclude? What are the risks and how do you minimise them? What are the benefits?

Ms Cliffe suggests that single-mindedness is essential. Producers who ring up wanting to discuss 'ideas' are politely told to return when they have an actual commission. Then 'we consider the relevance of the story to the hospital's aims'.

Frequent requests are for stories about transplants, conjoined twins, animal therapy and cystic fibrosis. They are dealt with on merit.

Certain potentially exploitative areas are strictly no-go. 'We do not allow filming of children in our psychological unit or those with eating disorders

or HIV.'

Other Great Ormond Street rules include no filming of patients or staff without written consent and no filming of personnel work where individuals are discussed.

Brighton Healthcare trust refuses to allow any filming which would - inadvertently or otherwise - endorse a commercial product.

Matt Tee, head of public relations at Guy's and St Thomas' Hospital trust, has a rule of thumb that where television exposure can raise the profile of the trust in a positive way, it is generally worth going the extra yard to accommodate the crews.

'When I first came here the general attitude was that television crews are a nuisance.

'But there's great value for us when the BBC health correspondent does a piece on nurses' pay, say, with the hospital in the background. It says: 'This is an important hospital, the BBC comes here.'

'However, if people just want to bring a crew down and get some live shots of a hospital - and no one will recognises it as Guy's or Tommy's - then I'm not interested,' says Mr Tee.

Once a trust has decided to go ahead with a television film, careful planning is required to agree terms.

Trusts will spend weeks negotiating detailed written agreements with crews to establish ethical and practical ground-rules for filming.

It is also important to plan for the aftermath.

Brighton Healthcare trust received over 190 letters and phone calls after a Tomorrow's World item on a new cataract operation filmed at

the trust.

A short piece on Newsroom South East about a new treatment for fibroids resulted in Guy's and St Thomas' trust being inundated with inquiries from women wanting the treatment.

The trust had set up a dedicated helpline but could not provide treatment for the numbers of women wanting it.

So the item raised issues of waiting lists and rationing, publicity that could later backfire on

the trust.

'It can cause despair. You get lots of people excited and ringing up, yet you know that you can't meet demand, that there are waiting lists.

But you have to balance that against letting the public know about new treatments,' explains

Mr Tee.

The benefits can outweigh the risks. The Royal Hospitals trust, London, agreed to a six-part fly-on-the-wall series with Carlton Television, partly because it wanted to combat the effect on staff recruitment caused by a series of critical press articles on the St Bartholemew's Hospital closure.

The series did not solve the trust's recruitment problem but did help remove the perception among potential recruits that Bart's was 'closing tomorrow'.

Press officer Sara Moseley says: 'In terms of stemming negative publicity it was a success.'

There are also marginal financial spin-offs from television exposure. Ms Cliffe agrees that Children's Hospital did 'help' with fundraising at Great Ormond Street although this is not easily quantifiable.

Guy's and St Thomas' does not charge for news crews or documentaries. But it is often asked if crews can film views of London from the top of Guy's tower - for which it charges 75 plus. And it charges feature film crews 2,000 for a weekend's shoot.

There is little sign of television's obsession with health abating.

George Cathro, a producer with Hand Pickt Productions, points out that new digital technology will bring with them many more channels - creating a voracious demand for both new and tried and tested material.

The challenge will be for television to look into relatively unexplored areas such as geriatric care and mental health. These will throw up tough but not necessarily insurmountable ethical problems about consent and patient confidentiality.

'Television producers are going to have to look at areas which have not been done before. But if they are done responsibly I do not see any reason why programmes cannot be made in those areas,' says Mr Cathro.