BACKGROUND: The efficacy of triple-drug antiretroviral regimens in the treatment of patients infected with HIV has been established in several randomized clinical trials. However, the effectiveness of these new regimens in patient populations outside clinical trials remain unproven. This study compared mortality and AIDS-free survival among HIV-infected patients in British Columbia who were treated with double- and triple-drug regimens. METHODS: The authors used a prospective, population-based cohort design to study a population of HIV-positive men and women 18 years or older for whom antiretroviral therapy was first prescribed between Oct. 1, 1994, and Dec. 31, 1996; all patients were from British Columbia. Rates of progression from the initiation of antiretroviral therapy to death or to diagnosis of primary AIDS were determined for patients who initially received an ERA-II regimen (2 nucleoside analogue reverse transcriptase inhibitors [NRTIs] including lamivudine or stavudine, or both) and for those who initially received an ERA-III regimen (triple-drug regimen consisting of 2 NRTIs and a protease inhibitor [indinavir, ritonavir or saquinavir] or a non-NRTI [nevirapine]). RESULTS: A total of 500 men and women (312 receiving an ERA-III regimen and 188 an ERA-III regimen) were eligible. Patients in the ERA-III group survived significantly longer than those in the ERA-II group. As of Dec. 31, 1997, 40 patients had died (35 in the ERA-II group and 5 in the ERA-III group), for a crude mortality rate of 8.0%. The cumulative mortality rates at 12 months were 7.4% (95% confidence interval [CI] 5.9% to 8.9%) for patients in the ERA-II group and 1.6% (95% CI 0.7% to 2.5%) for those in the ERA-III group (log rank p = 0.003). The likelihood of death was more than 3 times higher among patients in the ERA-II group (mortality risk ratio 3.82 [95% CI 1.48% to 9.84], p = 0.006). After adjustment for prophylaxis for Pneumocystis carinii pneumonia or Mycobacterium avium infection, AIDS diagnosis, CD4+ cell count, sex and age at initiation of therapy, the likelihood of death among patients in the ERA-II group was 3.21 times higher (95% CI 1.24 to 8.30, p = 0.016) than in the ERA-III group. Cumulative rates of progression to AIDS or death at 12 months were 9.6% (95% CI 7.7% to 11.5%) in the ERA-II group and 3.3% (95% CI 1.8% to 4.8%) in the ERA-III group (log rank p = 0.006). After adjustment for prognostic variables (prophylaxis for P. carinii pneumonia or M. avium infection, CD4+ cell count, sex and age at initiation of treatment), the likelihood of progression to AIDS or death at 12 months among patients in the ERA-II group was 2.37 times higher (95% CI 1.04 to 5.38, p = 0.040) than in the ERA-III group. INTERPRETATION: This population-based cohort study confirms that patients initially treated with a triple-drug antiretroviral regimen comprising 2 NRTIs plus protease inhibitor or a non-NRTI have a lower risk of morbidity and death than patients treated exclusively with 2 NRTIs.