Peptic ulcer mortality in Western Europe, Japan, and the United States is characterized by marked temporal changes which suggest that environmental factors are important in the etiology of gastric and duodenal ulcer. The present study examines at what age these factors start becoming effective. When the average age-specific death rates from 18 countries are plotted vs the year of birth, the fall of gastric and duodenal ulcer mortality related to successive birth-cohorts is continuously preserved in the age group of 15-19 years. Although the younger age groups also display a decline in ulcer mortality, it is not possible to discern unequivocally whether they follow the temporal pattern of birth-cohort risks. In the second analysis, the number of deaths from each individual country is accumulated over time, and the geographic variation in the age-specific mortality is compared for these countries. The death rates of consecutive age groups from different countries change in a parallel manner. The linear relationships between each two consecutive age groups start at the age of 5 years in gastric ulcer and at the age of 15 years in duodenal ulcer. These findings again suggest that the determinants for the risk of dying from gastric and duodenal ulcer begin to act at an age lower than 15. The contention of environmental factors starting to act before the age of 15 would remain valid, even if factors unrelated to etiology, such as different reporting for childhood and adult mortality, contributed to the different patterns of childhood and adult death rates. However, the actual cut-off age of gastric and duodenal ulcer would then be shifted towards younger ages.