The laptop was stolen from a locked room at a trust office used by the payroll department. It contained the names, addresses and bank account details of 10,000 staff from the acute trust and Cornwall and Isles of Scilly primary care trust, as well as personal details of 2,000 staff from Cornwall Partnership trust.
The data is normally stored on a secure server, but for unspecified reasons had been downloaded to the laptop, which was left overnight in the locked and alarmed office.
Royal Cornwall Hospitals trust spokesman said: 'We have initiated an internal inquiry to answer why the data had been downloaded and how the theft occurred.
'We have taken advice from the NHS Counter Fraud and Security Management Service and the police, and informed all members of staff by e-mail, intranet and via their line managers. We have advised them to let their banks know.'
Police inquiries are continuing.