To define patterns of care for the local therapy of stage I and II breast cancer and to identify factors used to select patients for breast-conserving therapy (BCT). A convenience sample of 16,643 patients with stage I and II breast cancer treated in 1994 was obtained from hospital-based tumor registries. Histologic variables were determined from original pathology reports. BCT was performed in 42.6% of patients. Multivariate analysis demonstrated that living in the Northeast United States (odds ratio [OR], 2.48; 95% confidence interval [CI], 2.16 to 2.84), having a clinical T1 tumor (OR, 2.51; 95% CI, 2.27 to 2.78), and having a tumor without an extensive intraductal component (OR, 2.07; 95% CI, 1.81 to 2.37) were the strongest predictors of breast-conserving surgery. Radiation therapy was given to 86% of patients who had breast-conserving surgery. Age less than 70 years was the most significant predictor of receiving radiation (OR, 2.11; 95% CI, 1.77 to 2.25). Tumor variables did not correlate with the use of radiation, but favorable tumor characteristics were associated with the use of breast-conserving surgery. Despite strong evidence supporting the use of BCT, the majority of women continue to be treated with mastectomy. Predictors of the use of BCT do not correspond to those suggested in guidelines.