Background: Differences in breast cancer incidence time trends can result from changes in ascertainment, new diagnostic codes, or possibly changes in underlying risk factors. Methods: Female breast cancer incidence data between 1985 and 2004 were obtained from the Pennsylvania Cancer Registry. Joinpoint regression was applied to characterize time trends of age-specific, race-specific, and histology-specific breast cancer incidence. Estimated annual percent change (APC) was calculated. Spatial analysis was applied to detect spatial clusters of county-specific incidence for breast cancer in Pennsylvania. Results: The age-adjusted incidence of invasive breast carcinoma and breast carcinoma in situ was higher in white women than in black women. Invasive breast carcinoma incidence began to decrease significantly in 2001 (APC -3.0%) among white women but has been stable among black women since 1987. Among white women, the age-adjusted incidence of ductal carcinoma in situ increased significantly from 1985 to 1999 but was stable for lobular carcinoma in situ. Among black women, the incidence for both ductal and lobular carcinoma in situ increased significantly over time. For women under the age of 40, breast carcinoma in situ incidence increased significantly over time (APC 4.5% and 10.0% in white women and black women, respectively, 1985-2004). Young black women had a higher incidence of both invasive breast carcinoma and breast carcinoma in situ compared to young white women. Conclusions: Although the increase in breast carcinoma in situ is partly explained by improved diagnosis and screening, other risk factors should be considered. In addition, factors responsible for higher breast cancer rates among younger black women and women living in urban areas should be carefully assessed.