The age-standardized breast cancer mortality rate for Canadian females increased only slightly during the period 1941-1971. In contrast, the age-standardized breast cancer incidence rate for Saskatchewan females more than doubled during the same period. In the interval 1954-1973 there were significant increases in the age-specific mortality rates for females 50-64 years old and in the age-specific incidence rates for five of the six age groups in the 45 to 74 year range. Breast cancer mortality rates during 1970-1972 varied considerably by province. Age-specific rates in the 30-69 year range were significantly and negatively correlated with the proportions by province of married women first married at age 15-19. In Canada and Saskatchewan, respectively, the age-specific breast cancer mortality and incidence rates for females 40-59 years old were significantly and negatively correlated with cohort fertility rates at age 20-24. These data support the hypothesis that early pregnancy protects against breast cancer. The sharp decline in fertility rates since 1960 may result in an increased incidence of breast cancer in the future.