Clinicopathological features of infection of the nervous system by cytomegalovirus (CMV) in 31 patients with the acquired immune deficiency syndrome (AIDS) are reviewed. Neuropathology was variable, ranging from rare isolated CMV inclusions in brain without associated inflammation or necrosis, to severe necrotizing ependymitis and meningoencephalitis. In 1 patient, CMV had produced a necrotizing meningoradiculitis which presented clinically as ascending paralysis. In the brains and spinal cords of 6 patients, evidence of human immunodeficiency virus (HIV) infection of neural parenchyma was seen in close proximity to CMV infection. Both viruses individually or together were associated with low grade (microglial nodule) encephalitis. In retrospect, the diagnosis of CMV had been a difficult one to make clinically in neurologically impaired patients with AIDS. The results suggest that CMV may also localize in the nervous system without significant clinical sequelae. Imaging studies and analysis of cerebrospinal fluid revealed abnormalities in many patients, but none of them (short of culture of CMV itself in two cases) appeared to be specific to this neurological complication of the immunodeficiency.