We examined cognitive performance in 72 HIV-1 infected patients and 34 controls. None of the patients had opportunistic infections or unusual neoplasms of the central nervous system (CNS). Factors other than HIV-1 known to cause cognitive decline were excluded from both groups. Cognitive functioning analysed with special emphasis on the severity of HIV infection was related to neuroradiological and immunological findings. In patients with AIDS-related complex (CDC IVa) or AIDS (CDC IVc,d), a deterioration of memory as well as cognitive speed and flexibility was detected. Furthermore, memory deficits were associated with central cerebral and infratentorial atrophy in those patients, while no association was found between cognitive deficits and immunological abnormalities. Patients at CDC stages II or III showed slight association between altered cognitive speed and flexibility and elevated leukocyte count, suggesting a subclinical CNS disease already at early stages of HIV infection.