The present study attempts to estimate the prevalence of Aids Dementia Complex assessed by neuropsychological testing in Norwegian patients with AIDS using a clinical control group with acute leukemia and an asymptomatic HIV-positive group as reference groups. Newly diagnosed patients with AIDS and not receiving zidovudine or other anti-viral drugs, patients with asymptomatic HIV infection, and newly diagnosed patients with acute leukemia, were studied with a battery of neuropsychological tests. Speeded tests and composite non-verbal measures discriminated significantly between groups. The results indicate higher than 50% prevalence of ADC in newly diagnosed Norwegian patients with AIDS. Our findings indicate that the AIDS population may contain two distinct groups, a subgroup with ADC and a subgroup with persistently normal neuropsychological function. The group with asymptomatic HIV infection showed normal neuropsychological performance.