In early 1987, immunizations were introduced to an isolated area of eastern Senegal where there had previously been no regular immunizations. Since immunizations were the only change introduced in the area during this period, this allowed the authors to study the impact of immunizations of mortality in different age groups and the possible interaction with sex differences in mortality. They compared mortality rates for the 6 years before and the 6 years after the introduction of immunization. Neonatal mortality declined 31% (95% confidence interval (CI) 17 to 43); between 1 and 8 months of age, the reduction was 20% (95% CI -2 to 37); and between 9 and 59 months of age, mortality declined 48% (95% CI 39 to 56). Excluding acute measles deaths, the reduction was 16% (95% CI -8 to 35) between 1 and 8 months of age and 32% (95% CI 20 to 43) between 9 and 59 months of age. The decline was stronger in villages that maintained high coverage after the initial national campaign, whereas mortality increased again in the villages where the coverage declined. Since the reduction in mortality was most marked after 9 months of age, measles immunization is likely to have been the most important vaccination. Both female and male mortality declined but not equally quickly. The reduction in mortality in the neonatal period was significantly greater in males than in females, resulting in an increase in the female/male mortality ratio from 0.64 (95% CI 0.50 to 0.83) to 0.96 (95% CI 0.71 to 1.30), p = 0.04. After 9 months of age, the reduction in mortality was somewhat greater in females than in males, resulting in a decrease in the female/male mortality ratio from 1.04 (95% CI 0.85 to 1.28) to 0.79 (95% CI 0.62 to 1.02), p = 0.10.