We undertook this study to determine the incidence proportion of and risk factors for AIDS patients diagnosed with human immunodeficiency virus (HIV) dementia, central nervous system (CNS) toxoplasmosis, and cryptococcal meningitis. A historical cohort of 487 consecutive inpatients with AIDS treated by San Francisco General Hospital inpatient and outpatient services entered the study. We abstracted all available records for demographic information, diagnoses, and dates of death and estimated the incidence proportion of AIDS patients diagnosed with major CNS complications using the Kaplan-Meier method. We used the Cox proportional hazards model to analyze the effect of demographic factors on the hazard (risk per unit time) of diagnosis with these CNS conditions. The estimated incidence proportion of patients diagnosed with HIV dementia within 1 and 2 years of AIDS diagnosis increased from 0.10 to 0.18. Corresponding proportions were 0.10 and 0.19 for CNS toxoplasmosis and 0.10 and 0.14 for cryptococcal meningitis. Only HIV dementia was independently associated with increasing age at AIDS diagnosis (relative hazard [RH] of 2.75 for ages 41-50 [95% confidence interval, 1.08-6.98]; RH of 4.73 for ages > 50 [95% confidence interval, 1.41-15.87]) and with injection drug use (RH of 2.03; 95% confidence interval, 1.19-3.47). HIV dementia, CNS toxoplasmosis, and cryptococcal meningitis are about equally common complications in patients with AIDS, but only HIV dementia is associated with increasing age at AIDS diagnosis and injection drug use.