As more women obtain screening mammograms regularly and at younger ages, the diagnosis of breast carcinoma in situ becomes more frequent. To examine whether risk factors for carcinoma in situ correspond with risk factors for invasive breast cancer, we analyzed data from a population-based case-control study conducted in 1988-1990. We identified newly diagnosed cases of carcinoma in situ (n = 301) and invasive breast cancer (n = 3789) in women 18-74 years of age from Wisconsin's statewide tumor registry. Cases and population controls (n = 3999) completed structured telephone interviews. Overall, associations with risk of carcinoma in situ in relation to many reproductive life-style risk factors were similar to those associated with risk of invasive disease. Women who reported a family history of breast cancer had a 2-fold elevated risk of carcinoma in situ (odds ratio, 2.67; 95% confidence interval, 2.00-3.57). Personal history of benign biopsied breast disease also increased risk of carcinoma in situ (odds ratio, 2.19; 95% confidence interval, 1.62-2.95). Subgroup analysis suggested that high vitamin A intake and high alcohol intake may be associated with risk of ductal but not lobular carcinoma in situ. These data support the presence of common risk factors between in situ and invasive breast cancer.