Up to 4,000 patients at Stockport Primary Care Trust (PCT) have not been told their medical details have been lost, it has been revealed.
The data stick was clipped around a staff member's neck
A data memory stick holding patients' names, dates of birth, NHS numbers and GP details has been missing since December, the PCT said.
Patients have not been informed as the PCT believes the information cannot be used by identity fraudsters.
The Department of Health and GPs have been notified.
The data memory stick was lost by a staff member who clipped it onto a cord around her neck before walking to her desk from her car.
In a statement, PCT chief executive Richard Popplewell said: "Immediate steps were taken to search for the drive by retracing the path of the staff member but the drive has not been found.
"The loss was an accident rather than any systematic failing in management and governance.
"It is extremely likely that the data was lost in circumstances in which it was irrecoverable and there was a balance to be drawn between being open with patients and on the other hand avoiding unnecessarily upsetting them."
The memory stick contained the details of patients who suffered from chronic obstructive pulmonary disease, asthma, heart failure, coronary heart disease, diabetes or epilepsy.
Mike Lappin, chairman of Stockport PCT patient forum, said: "Those involved should have been told immediately if information about them has gone missing - it is just good manners."
The PCT has also revealed that diaries containing patients' names and addresses were stolen from staff cars in two separate incidents in June.