We evaluated the incidence of AIDS dementia complex (ADC) in groups of patients who acquired infection through different risk behaviours, and attempted to evaluate the possible role of zidovudine (AZT) treatment in preventing or delaying the onset of ADC. The Italian National AIDS Registry was used to study patients with AIDS for whom ADC was reported as an index disease. Relative risk of presenting ADC between different patient categories has been determined. Logistic regression was used to analyse temporal trends in the proportion of AIDS cases presenting with ADC. Of the 6466 cases reported between August '87 and August '90, ADC was seen in 640 (9.9%). I. V. drug addicts had twice the risk (estimated odds ratio: 1.9; 95% confidence interval: 1.5-2.6, p less than 0.001), compared to homo/bisexuals, of presenting with ADC. There is significant evidence (p less than 0.0001) that after a progressive increase in the period '87-'89, it began a definite decrease in the monthly proportion of ADC cases, starting August '89. AZT was introduced in Italy in July 1987 for patients with AIDS or advanced ARC. The incidence of AIDS dementia complex at the moment of AIDS diagnosis in our population of patients, began to decline 24 months after the introduction of systematic AZT treatment in Italy. This could have been due to inhibition of HIV replication in the Central Nervous System among patients who initiated AZT-treatment before developing full-blown AIDS.