The influence of treatment with zidovudine (ZDV) and other factors on long-term survival of AIDS patients was analyzed in a cohort of 629 adults. A total of 434 (69%) were diagnosed before ZDV became routinely available in Spain (December 1987) or refused the drug, while the remaining 195 (31%) received ZDV (starting at a dose of 750-1,200 mg/day). A total of 412 (65.5%) were parenteral drug addicts and 217 (34.5%) male homosexuals. Two hundred thirty-two (36.9%) presented with a tuberculosis, 303 (48.2%) with other opportunistic infections, 69 (11%) with Kaposi's sarcoma, and the remaining 25 (4%) with a lymphoma. By December 1990, 251 (39.9%) of the 629 have already died with a cumulative survival probability of 50.6% after 2 years (45.3-55.9%; 95% confidence interval). When patients receiving ZDV were compared with those untreated, the estimated survival probability was significantly (p less than 0.0001) higher (89% vs. 59% after 1 year, 69% vs. 48% after 2 years, and 55% vs. 40% after 3 years). Moreover, treatment with ZDV (p less than 0.0001) together with being less than 45 years old (p less than 0.0001), being a parenteral drug addict (p = 0.016), and presenting with tuberculosis (p less than 0.0001) were the factors selected by the multivariate analysis as independently improving the prognosis. In conclusion, adult AIDS patients (homosexual or drug addicts) may benefit from treatment with ZDV, at least during 3 years.