Infection with human immunodeficiency virus (HIV) is routinely and easily diagnosed with use of enzyme immunoassay (EIA) test kits. We describe an unusual patient who developed AIDS despite testing negative for antibodies to HIV 35 times over a 4-year period. HIV infection was confirmed by the results of p24-antigen assays and polymerase chain reaction amplification of proviral DNA. Sequence analysis of the virus demonstrated that it was closely related to a strain obtained from the patient's sexual partner. The explanation for this patient's persistently negative EIA results is unclear. However, this case does suggest that physicians who treat patients with AIDS-defining conditions but for whom standard HIV antibody testing is negative should consider the possibility that HIV infection is present and may be identified by additional testing procedures.