OBJECTIVE: To assess how HIV infection and AIDS (HIV/AIDS) impacts on mortality rates for gay and bisexual men. METHODS: Vital statistics data were obtained for a large Canadian urban centre from 1987 to 1992. Three scenarios were utilized with assumed proportions of gay and bisexual men of 3%, 6% and 9% among the male population age 20 years. For each scenario, non-HIV deaths were distributed according to the assumed proportion of the total population (3%, 6% or 9%) but 95% of HIV deaths were distributed to gay and bisexual men as this is the proportion of AIDS cases in gay and bisexual men in this centre. The main outcome measures of interest were age-specific patterns of death, life expectancy and life expectancy lost due to HIV/AIDS at exact age 20 years, and the probability of living from age 20 to 65 years. RESULTS: Estimates of the mid-period gay and bisexual population ranged from 5406 to 16,219 for the three scenarios, and total deaths in these men from 953 to 1703. Age-specific mortality was significantly higher for gay and bisexual men than all men aged 30-44. Life expectancy at age 20 for gay and bisexual men ranged from 34.0 years to 46.3 years for the 3% and 9% scenarios respectively. These were all lower than the 54.3 year life expectancy at age 20 for all men. The probability of living from age 20 to 65 years for gay and bisexual men ranged from 32% for the 3% scenario, to 59% for the 9% scenario. These figures were considerably lower than for all men where the probability of living from 20 to 65 was 78%. CONCLUSION: In a major Canadian centre, life expectancy at age 20 years for gay and bisexual men is 8 to 20 years less than for all men. If the same pattern of mortality were to continue, we estimate that nearly half of gay and bisexual men currently aged 20 years will not reach their 65th birthday. Under even the most liberal assumptions, gay and bisexual men in this urban centre are now experiencing a life expectancy similar to that experienced by all men in Canada in the year 1871.