The Russian government has confirmed spreadsheets containing private details of Covid-19 patients from Moscow have been published online.
An official has said an internal inquiry found a mistake had been made rather than hackers being to blame.
Readovka, the news site that revealed the leak, says the files indicate official statistics may have under-reported the true number of cases.
It says 300,000 names are listed alongside diagnoses and test results.
Medical insurance numbers, home addresses, telephone numbers and passport details were also featured alongside other personal information.
Current Time - a news network led by Radio Liberty in co-operation with Voice of America - has also carried out checks of its own on the data to confirm the details matched those of real people.