Few data exist on risk factors for in situ breast carcinoma. We examined risk factors for in situ breast carcinoma in data from two population-based case-control studies of breast cancer conducted among female residents of Los Angeles County. Cases with in situ or invasive disease were identified through the cancer registry for Los Angeles Country in the 1980s. We included all cases ages 40 years or younger diagnosed over a 5.5-year period and all cases ages 55-64 years diagnosed over a 3-year period. Control subjects were individually matched to cases by age (+/-3 years), neighborhood of residence, and, for younger controls, parity (nulliparous versus parous). The analysis included 233 cases with in situ cancer, 2057 cases with invasive cancer, and 2203 controls. Odds ratios (ORs) and 95% confidence intervals (CIs) were adjusted for screening mammography and established risk factors. In general, risk factors for in situ breast cancer were similar to those for invasive disease in this population. In premenopausal women, however, the risk of in situ breast cancer decreased with increasing body mass index, whereas for invasive disease, body mass index was unrelated to risk. In addition, in postmenopausal women with known age at menopause, use of unopposed estrogen replacement therapy was associated with increased risk of in situ disease [OR for ever use of estrogen alone was 1.60 (95% CI, 1.00-2.58)], whereas for invasive disease the OR was 1.23 (95% CI, 1.00-1.50). A similar difference was seen for combined hormone replacement therapy. Unmeasured increased screening among estrogen or combined replacement hormone users compared with nonusers could account for some of the association of in situ breast cancer risk with hormone replacement use.