Chemotherapy and hormone therapy prolong disease-free and overall survival for patients with breast cancer in the clinical-trial setting, but it is not clear if this translates into a benefit on a population basis. It is also not clear if surgical caseload has any influence on survival. We used cancer-registry data from 12,861 patients with breast cancer treated in Yorkshire, UK, between 1979 and 1988, and found that patients of surgeons with higher rates of usage of chemotherapy and hormone therapy (regional mean usage 9.3%, range 0-46%) had prolonged survival. There was considerable variation in survival of breast cancer patients between surgeons, but their rate of use of chemotherapy and hormone therapy explained about 26% of this survival variation. Had the practice of the surgeons with the better outcomes been used by all treating clinicians, 5-year survival would have increased by about 4-5%. Examination of differences in survival as a function of consultant caseload demonstrated poorer results amongst those surgeons treating less than 30 new cases of breast cancer per year (risk ratio [95% CI] for treating > 30 compared with < 10 = 0.85 [0.77-0.93]). We recommend that patients with breast cancer be dealt with only by clinicians who see more than 30 new cases per year and who have a full range of treatment options available within a multidisciplinary setting.