OBJECTIVES: To compare two population-based birth cohorts to assess trends in infant mortality rates and the distribution of relevant risk factors, and how these changed after an 11-year period. METHODS: Data from two population-based prospective birth cohorts (1982 and 1993) were analyzed. Both studies included all children born in a hospital (> 99% of all births) in the city of Pelotas, Southern Brazil. Infant mortality was monitored through surveillance of all maternity hospitals, mortality registries and cemeteries. RESULTS: There were 5 914 live-born children in 1982 and 5 249 in 1993. The infant mortality rate decreased by 41%, from 36.0 per 1 000 live births in 1982 to 21.1 per 1 000 in 1993. Socioeconomic and maternal factors tended to become more favorable during the study period, but there were unfavorable changes in birthweight and gestational age. Poverty, high parity, low birthweight, preterm delivery, and intrauterine growth restriction were the main risk factors for infant mortality in both cohorts. The 41% reduction in infant mortality between 1982 and 1993 would have been even greater had the prevalence of risk factors remained constant during the period studied here. CONCLUSIONS: There were impressive declines in infant mortality which were not due to changes in the risk factors we studied. Because no reduction was seen in the large social inequalities documented in the 1982 cohort, it is likely that the reduction in infant mortality resulted largely from improvements in health care.