The confidential personal health data of about 1.7 million New York City patients, hospital staffers and others were stolen in December, the city's Health and Hospitals Corp. reported Friday.

The medical files, which were stored on magnetic data tapes and extend back as long as 20 years, were stolen on Dec. 23 from an unlocked vehicle belonging to GRM Information Management Services, the city's medical-records vendor based in Jersey City, N.J.

The...