The records of 480 patients who received systemic therapy for advanced breast cancer in the University Hospital of South Manchester from 1975 to 1983 were examined. There were 264 with infiltrating duct carcinomas (IDC) and 33 with infiltrating lobular carcinomas (ILC) for whom the response to endocrine therapy was known. There were 92 responses (35%) among the IDC patients and 9 (27%) among the ILC patients. Sixty-seven per cent of IDC patients tested had steroid hormone receptor positive tumours compared to 90% for ILC (P less than 0.001). Comparison of survival from diagnosis, disease free interval and survival from relapse showed no significant differences between the two groups. Thus despite almost all ILC patients having hormone receptor positive tumours their survival was similar to that of IDC patients. This appears to be due to a lower than expected response rate to endocrine therapy. This is a further indication of the different biological characteristics of these two histological sub-types of breast carcinoma.