Trends in the incidence of stomach cancer among 52,604 patients notified to the National Swedish Cancer Registry in 1960 through 1984 were analysed. Age-standardized incidence rates declined throughout the period, from 47.1 to 24.6 per 10(5) in males and from 23.8 to 12.7 per 10(5) in females. Among males the decline in incidence was more pronounced in younger age strata, 35-54 years, while in females it reached a maximum at ages 70-74 years. In a multivariate analysis the age-cohort model adequately represented the data and there was no reason to separate the effects of the full age-period-cohort model. Compared with the cohort born in 1876-1884, the relative risk of developing stomach cancer was 0.46 (95% CI:0.44-0.48) in males and 0.39 (95% CI:0.37-0.42) in females born in 1906-1914. This supports the view that the declining incidence is due to a change in the exposure of the population to aetiological factors of stomach cancer and not to refinement of the diagnosis and classification of abdominal tumours. The results also imply that exposure to aetiological factors early in life together with a long induction period is of importance in the development of stomach cancer.