To characterize the recent decrease in breast cancer incidence in relation to earlier secular trends, we examined temporal trends in incidence rates by tumor size, stage, and estrogen receptor/progestin receptor (ER/PR) status and trends for in situ breast cancer among women at least 40 years old from 1975 through 2003 by means of incidence data from the nine oldest SEER cancer registry areas. The gene discussed is ESR1; the disease is breast carcinoma in situ.