The recent increase in incidence in situ breast cancer has been marked by a higher detection rate among white women. Although the increase in incidence may reflect the concomitant uptrend in mammographic screening, the lower proportion of cases among black women is of major public health concern. Time trends in the diagnosis of in situ breast cancer were evaluated in a population-based analysis of data accrued from the Metropolitan Detroit Cancer Surveillance System. The proportions of in situ cases detected among all women with breast cancer were measured annually between 1973 and 1987, and the average interval percentage changes were calculated for eight subgroups of women stratified by race and age at diagnosis. Although the proportions of in situ cancers were generally higher among white than black women, the greatest increase in average interval percentage change was observed in the oldest age category of black women. The disparity seen in younger black and white women suggests possible implications for breast cancer screening. From 1973 through 1987, the largest increase in diagnosis of in situ breast cancer occurred in black women older than 70.