HYPOTHESIS: Some risk factors associated with breast cancer may be more predictive of estrogen receptor (ER)- positive than ER-negative tumors. DESIGN: Survey of patients enrolled in a study of breast cancer risk factors. SETTING: Community population in a northern California county. PATIENTS: A total of 234 individuals diagnosed as having breast cancer between July 1, 1997, and June 30, 1999, reporting Marin County, California, residence and participating in a questionnaire regarding exposure to breast cancer risk factors. MAIN OUTCOME MEASURE: Diagnosis of ER-positive vs ER-negative breast cancer. RESULTS: Comparison between ER-positive and ER-negative cases showed several factors predictive of ER-positive tumors. In a multivariate model, years of hormone therapy use remained the most significant predictor of ER-positive disease. CONCLUSIONS: Patients diagnosed as having ER-positive breast cancer were more likely to have undergone hormone therapy. The excess of ER-positive breast cancers reported in Marin County could, therefore, in part, be related to hormone therapy.