Although neuropsychiatric abnormalities are common in subjects with the acquired immunodeficiency syndrome (AIDS), they are less frequent in asymptomatic human immunodeficiency virus (HIV) seropositive subjects. In contrast, others have reported high rates of electroencephalographic (EEG) abnormality among asymptomatic subjects. Here we report clinical and quantitative EEG findings across all stages of the disease in order to define when during the course of illness abnormalities are detectable. We studied 28 men with AIDS, 32 men with asymptomatic HIV infection, and 56 uninfected controls using clinical and quantitative EEG, measures of immunosuppression, and tests of neuropsychological performance. All were gay or bisexual without other significant risk factors for encephalopathy. We found very low rates of clinical EEG abnormality (less than 7%) among the asymptomatic HIV-infected group, a rate comparable to those of the uninfected group (7.1%). There were no differences between asymptomatic HIV-seropositive subjects and uninfected controls on quantitative EEG measures. Among AIDS patients 28.6% had abnormal clinical electroencephalograms. On quantitative measures, the greatest differences were found in the 6-10 Hz band, where AIDS patients had consistently increased absolute power, relative power, and coherence compared to the uninfected and asymptomatic seropositive groups. A subgroup (n = 9) of asymptomatic HIV-seropositive subjects had worsening performance on Trailmaking test, part B, at or after the time of recording. This subgroup had quantitative electroencephalographic measures similar to those of the AIDS patients and different from the remainder of the asymptomatic HIV-seropositive group.(ABSTRACT TRUNCATED AT 250 WORDS)