Hundreds of documents containing HIV and cancer test results have been found on a street in south-west London.
The test results are recognisable by a unique number
The 173 private medical documents, which were discovered in a motorbike bag near Kingston Hospital last Friday, were handed over to a local newspaper.
A Kingston Hospital spokeswoman said test results were only recognisable by a unique number but said all patients would receive written apologies.
She said an investigation, which may be assisted by the police, is under way.
Many of the documents which have been sent to Queen Mary's Hospital in Roehampton and the Roehampton Clinic, also included information on those attending conception and addiction clinics, as well as sexual disease and hepatitis test results.
"We take the loss of any patient information very seriously, and all the patients involved will be written to personally offering sincere apologies," the spokeswoman said.
She added that the hospital could not explain how or when the data protection breach occurred.
A spokesman for the London Strategic Health Authority (SHA) confirmed the matter would be dealt with by Kingston hospital, but added that the SHA would intervene if necessary during the investigation.
"We are keeping a close eye on what is going on as far as the investigation goes, but it is being dealt with at a local level," he said.
"We will step in later if any issues arise."