A group of 66 elderly women with primary breast cancer were treated with tamoxifen and followed for a minimum of 2 years. Of these, 32 whose disease remained controlled for the 2-year period were considered to have had a 'worthwhile' response, 27 in whom disease progressed were considered to have had an unsatisfactory result and seven opted for alternative treatment. By Union Internacional Contra la Cancrum (UICC) criteria, 14 women had a complete response, 20 a partial response, in five disease remained static and in 20 it progressed without response. Prediction of outcome after assessment according to UICC criteria at 3 and 6 months was unsatisfactory (19 and 34 of 59 correctly predicted respectively). Analysis of multiple tumour measurements over 12 weeks was no better (33 of 59 correctly predicted). Immunocytochemical assay of fine-needle aspirates for oestrogen receptor (ER) provided a better predictor (38 of 47 correct) and the difference in survival between patients with and without ER activity was significant (P < 0.001). Conventional assessments of response at 3 and 6 months are unsatisfactory for judging the long-term benefit to the patient. ER status is the best predictor of response and outcome.