Clinical trials have established the efficacy of antiretroviral therapy with double- and triple-drug regimens for individuals infected with the human immunodeficiency virus (HIV), but the effectiveness of these regimens in the population of patients not enrolled in clinical trials is unknown. To characterize survival following the initiation of antiretroviral therapy among HIV-infected individuals in the province of British Columbia. Prospective, population-based cohort study of patients with antiretroviral therapy available free of charge (median follow-up, 21 months). Province of British Columbia, Canada. All HIV-positive men and women 18 years of age or older in the province who were first prescribed any antiretroviral therapy between October 1992 and June 1996 and whose CD4+ cell counts were less than 0.350 x 10(9)/L. Rates of progression from initiation of antiretroviral therapy to death or a primary acquired immunodeficiency syndrome (AIDS) diagnosis for subjects who initially received zidovudine-, didanosine-, or zalcitabine-based therapy (ERA-I) and for those who initially received therapy regimens including lamivudine or stavudine (ERA-II). A total of 1178 patients (951 ERA-I, 227 ERA-II) were eligible. A total of 390 patients died (367 ERA-I, 23 ERA-II), yielding a crude mortality rate of 33.1%. ERA-I group subjects were almost twice as likely to die as ERA-II group subjects, with a mortality risk ratio of 1.86 (95% confidence interval [CI], 1.21 -2.86; P=.005). After adjusting for Pneumocystis carinii and Mycobacterium avium prophylaxis use, AIDS diagnosis, CD4+ cell count, sex, and age, ERA-I participants were 1.93 times (95% CI, 1.25-2.97; P=.003) more likely to die than ERA-II participants. Among patients without AIDS when treatment was started, ERA-I participants were 2.50 times (95% CI, 1.59-3.93; P<.001) more likely to progress to AIDS or death than ERA-II participants. The HIV-infected individuals who received initial therapy with regimens including stavudine or lamivudine had significantly lower mortality and longer AIDS-free survival than those who received initial therapy with regimens limited to zidovudine, didanosine, and zalcitabine.