The health records of nearly 11,000 patients are on a laptop stolen from the Wolverhampton home of a GP.
The doctor, who has not been named, works at the Castlecroft Medical Practice in the city.
The information was not encrypted to make it difficult to access, as health guidelines say it should be.
But a practice spokesman said the information was protected with a complex password system which would be difficult to crack.
The information on the laptop included names, dates of birth, addresses, contact details and confidential records.
The practice has written to all of its patients to inform them of the matter.
Senior partner Dr Peter Wagstaff said the practice was treating the matter seriously.
Police have warned that identity theft could now be an issue
"Though not encrypted, the confidential information on the laptop was protected by a complex password system, which only a person with specialist computer knowledge would be able to crack," he said.
Department of Health guidelines state any confidential information about patients must be stored in safe and secure environments.
Jon Crockett, chief executive of Wolverhampton Primary Care Trust, said an investigation would be carried out but added that the doctor had been a victim of crime.