Between September 1, 1985 and September 1, 1989, 110 premenopausal patients with estrogen-receptor-positive, stage-II breast cancer were randomized to receive, as adjuvant treatment following radiotherapy, either cyclophosphamide, methotrexate, and 5-fluorouracil (CMF) + tamoxifen (T) + bromocriptine (B) bases or CMF + T only. Preoperative serum prolactin (PRL) levels or PRL-receptor status of the tumor were not available, but before the commencement of adjuvant therapy serum PRL levels were measured in all patients and found to be high in 28. The local (LR) and distant (DM) metastasis-recurrence rates were lower in patients given bromocriptine (CMF + T + B) (LR, 5.7%; DM, 10.9%) than in those not given bromocriptine (CMF + T) (LR, 10.9%; DM, 27.2%); these findings, however, were not significant (P > 0.05). In the 28 hyperprolactinemic patients the metastasis-recurrence rate (17/28) was higher (P = 0.0001) and disease-free survival was shorter (P = 0.001) than in the 80 normoprolactinemic patients. It was also demonstrated that the disease-free survival was longer (P = 0.009) and the metastasis-recurrence rate was lower (6/12) in hyperprolactinemic patients who received bromocriptine (CMF + T + B) than in hyperprolactinemic patients who did not (CMF + T) (11/16), while there was no difference in metastasis-recurrence rates and disease-free survival between the two treatment groups among normoprolactinemic patients. These results encourage further investigation of the action of bromocriptine adjuvant base. Baseline serum PRL measurement and tumor PRL-receptor determination could be valuable tools to identify the appropriate cases for antiprolactinemic treatment.