Isolation of the human immunodeficiency virus (HIV) has been attempted from the cerebrospinal fluid (CSF) of 63 subjects at different stages of HIV infection, including asymptomatic carriers and patients with or without neurologic or psychiatric complications. In addition blood was collected from 40 of these subjects for virus isolation. HIV could be isolated from the CSF at all clinical stages with an overall frequency of 40%. In contrast, the frequency of HIV isolation from the blood was lower (32%) at the early stages of infection than in patients with severe disease (77%). HIV isolation from the CSF was more frequently positive in patients with neurologic or psychiatric complications than in patients showing no such disturbances (48 and 32%, respectively).