Of 70 autopsied patients with the acquired immune deficiency syndrome (AIDS), 46 suffered progressive dementia that was frequently accompanied by motor and behavioral dysfunction. Impaired memory and concentration with psychomotor slowing represented the most common early presentation of this disorder, but in nearly one half of the patients either motor or behavioral changes predominated. Early motor deficits commonly included ataxia, leg weakness, tremor, and loss of fine-motor coordination, while behavioral disturbances were manifested most commonly as apathy or withdrawal, but occasionally as a frank organic psychosis. The course of the disease was steadily progressive in most patients, and at times was punctuated by an abrupt acceleration. However, in 20% of patients a more protracted indolent course was observed. In the most advanced stage of this disease, patients exhibited a stereotyped picture of severe dementia, mutism, incontinence, paraplegia, and in some cases, myoclonus. The high incidence and unique clinical presentation of this AIDS dementia complex is consistent with the emerging concept that this complication is due to direct brain infection by the retrovirus that causes AIDS.