The authors enrolled 1,029 patients with CD4 counts </= 200/microL and no CNS AIDS-defining events (CNS-AIDS) between January 1993 and December 1998. The primary end point was the first appearance of CNS-AIDS. Three different periods and different antiretroviral regimens were considered. During the median follow-up of 329 days, 144 patients (9.5%) developed CNS-AIDS. The independent predictors were CD4 counts and therapy. All regimens decreased the risk of CNS-AIDS. Highly active antiretroviral therapy led to a 95% risk reduction in comparison with untreated patients.