Sixty-seven patients with different stages of human immunodeficiency virus (HIV) infection (47 CDC group IV, 20 CDC groups II or III) were followed prospectively for a median of 18 months with neurological examination, magnetic resonance imaging (MRI), and computerized tomography (CT) to evaluate the incidence of the AIDS dementia complex (CDC definition) and other neurological complications. Ten patients developed CNS opportunistic infection or malignancy. Among the remaining 57 patients, 12 of 37 (32%) belonging to CDC group IV, and 1 of 20 (5%) belonging to CDC groups II/III developed the AIDS dementia complex (p = 0.03). MRI white matter lesions occurred in 32% of CDC group IV patients and 5% of CDC groups II/III patients (p = 0.03). The corresponding figures for brain atrophy at CT were 71% and 30% (p less than 0.01) and for neurologic signs 49% and 20% (p = 0.06). The development of the AIDS dementia complex was significantly associated with the occurrence of MRI white matter lesions and a CD4 cell count of less than 200 x 10(6)/l, whereas it was not statistical significantly associated with brain atrophy at baseline. It is concluded that the AIDS dementia complex is a common feature of late stage HIV infection. Brain atrophy occurs in a large percentage of HIV infected patients, but the clinical significance of this atrophy is not clear.