To examine possible causes for factitious human immunodeficiency virus (HIV) infection among patients in an HIV clinic. Retrospective chart review, a case-control study, and a survey of local hospital practices for documenting HIV infection. Clinical acquired immunodeficiency syndrome (AIDS) program at a municipal hospital. Seven patients with self-reported, undocumented HIV infection were identified as HIV seronegative after a mean of 9.2 months of care in our clinical AIDS program. The median CD4 count for these patients was 740 cells/mm3; 6 patients had a history of illicit narcotic use and clinical symptoms consistent with HIV disease. Compared with 70 randomly selected controls from HIV clinics, patients with factitious HIV infection had higher CD4 counts (difference, 519 cells/mm3; P < 0.001) and were more likely to have an HIV-infected sexual partner (odds ratio, 15.0; P = 0.005) and a history of a suicide attempt (odds ratio, 9.8; P = 0.02). Known cases of alleged HIV infection have occurred at 8 of the 10 other local hospitals surveyed. However, only 1 of the 10 hospitals routinely documented HIV infection in patients before initiating care. Limitations of the current serologic tests for HIV, the use of anonymous HIV testing, and recent reports of factitious HIV disease or immune deficiency syndromes that may mimic AIDS underscore the need for clear documentation of HIV infection before medical care is started.