Three major characteristics of human immunodeficiency virus (HIV) infection define HIV as neurotropic. 1) Clinically, distinct neurological syndromes are associated with HIV infection and 2) presence of the virus as well as 3) pathological changes can be demonstrated in the central nervous system. Spread of HIV to the brain seems to be the general rule. Virus expression appears to be restricted during the asymptomatic period but increases with severity of HIV infection. Whether this reflects the emergence of virus variants with increased replica-tive capacity in brain cells has yet to be elucidated.