Abstract PURPOSE: The San Francisco Bay Area has a history of high breast cancer incidence rates relative to the rest of the United States. For Marin County, where Bay Area rates are highest and, moreover, have continued to increase over time, age- and tumor-specific incidence trends were compared with the rest of the region. METHODS: The study included all white women diagnosed with invasive breast cancer in 1988 to 1997 in the five-county Bay Area (N = 19,807). Annual age-specific incidence rates and estimated annual percent changes (EAPCs) were calculated for women ages less than 45, 45 to 64, and greater than or equal to age 65. RESULTS: Women aged 45 to 64 from Marin County experienced a marked increase in breast cancer rates between 1991 and 1997 (EAPC = 8%, p = 0.02), regardless of disease stage or tumor histology. For the youngest and oldest women, no rate differences were observed by region or over time. CONCLUSIONS: This regional difference in trend by age did not appear to be due to screening mammography or environmental exposures. Cohort exposures to breast cancer risk factors, such as oral contraceptive and/or hormone replacement therapy use, may have contributed to these rate increases. Although the reasons remain unclear, the finding may signal a rising risk of breast cancer in this demographic group.