The breast cancer incidence rate (i.e., the detected number of new cases per 100,000 women) increased by 31% in western Washington State between the time periods 1974–1978 and 1986–1987. If this increase is largely due to earlier detection of cases through mammography, it is encouraging; otherwise (if it cannot be attributed to mammography alone), investigation of other factors may be needed. The observed increase, based on 18,559 documented breast cancer cases from the Seattle-Puget Sound Surveillance, Epidemiology, and End Results (SEER) registry, was compared with the predicted increase based on mammography utilization by year from a survey of 1,212 women in western Washington and on data from published studies regarding the impact of mammography on the detection rate for new breast cancer cases. Among women aged 45–64, all of the increase was associated with local stage disease and with tumors detected at a smaller size. Further, the 15% observed increase in incidence in this age group was less than the 20% increase predicted due to the utilization of mammography. However, the observed increase was approximately twice the predicted increase for women aged 65–74 (observed 57%, predicted 26%) and for women aged 25–44 (observed 29%, predicted 12%). Thus, all of the increase in detected breast cancer among 45–64 year old women appears to be explained by the increased use of mammography, while among older women and younger women there may be other contributing factors. The limitations and implications of this analysis are discussed. [J Natl Cancer Inst 82:1546–1552,1990]