Using a log-linear model, we examined the 59-year incidence trends (1932-1990) of breast cancer by age, sex, and birth cohort in Saskatchewan, Canada. The incidence of breast cancer showed a continuously increasing trend since the 1930s. The trend was age dependent--in general, the 60 years and older age group had greater increases in incidence rates than did younger age groups. The increase in incidence rates in the 45 years and older age group was consistent over time and the incidence rates for those younger than 45 years increased until the 1960s, with a decline in trend following the 1960s. Birth cohort was a significant factor in the incidence of breast cancer. The major birth cohort effect can be described for those born from 1852 to 1927--in every age group breast cancer incidence increased as the birth years advanced. For those born after 1927, the incidence of breast cancer was unchanged as the birth cohort changed and was sustained at a high level. Whether this phenomenon suggests that the risk factors for breast cancer ceased to increase over time needs to be confirmed by further studies.