Objective:To describe persons with HIV infection and AIDS but with persistently negative HIV antibody enzyme immunoassay (EIA) results.Design:Surveillance for persons meeting a case definition for HIV-1-seronegative AIDS.Setting:United States and Canada.Patients:A total of eight patients with seronegative AIDS identified from July 1995 through September 1997.Main outcome measures:Clinical history of HIV disease, history of HIV test results, and CD4 cell counts from medical record review; results of testing with a panel of EIA for antibodies to HIV-1, and HIV-1 p24 antigen; and viral subtype.Results:Negative HIV EIA results occurred at CD4 cell counts of 0-230×106/l, and at HIV RNA concentrations of 105000-7943000 copies/ml. Using a panel of HIV EIA on sera from three patients, none of the HIV EIA detected infection with HIV-1, and signal-to-cut-off ratios were ≤0.8 for all test kits evaluated. Sera from five patients showed weak reactivity in some HIV EIA, but were non-reactive in other HIV EIA. All patients were infected with HIV-1 subtype B.Conclusions:Rarely, results of EIA tests for antibodies to HIV-1 may be persistently negative in some HIV-1 subtype B-infected persons with AIDS. Physicians treating patients with illnesses or CD4 cell counts suggestive of HIV infection, but for whom results of HIV EIA are negative, should consider p24 antigen, nucleic acid amplification, or viral culture testing to document the presence of HIV.