Possible transmission of human immunodeficiency virus (HIV) infection during an invasive dental procedure was previously reported in a young woman (patient A) with acquired immunodeficiency syndrome (AIDS). Patient A had no identified risk factor for HIV infection and was infected with a strain of HIV closely related to that of her dentist as determined by viral DNA sequencing. A follow-up investigation has identified four additional patients of the dentist who are infected with HIV. Laboratory and epidemiologic investigation has been completed on three of these patients; two are infected with strains closely related to those of the dentist and patient A but not to strains from other persons residing in the same geographic area as the dental practice. The follow-up investigation included review of medical records of the dentist and interviews of former staff on the infection-control procedures of the dental practice. This report summarizes the findings of the investigation.