The evaluation of the biological status of cancer patients should not be limited only to investigation of immune reactivity, but should also include analysis of the endocrine condition, namely concerning those hormones which have appeared to be tumor growth factors, such as prolactin (PRL) for breast and prostate carcinomas. This statement is justified by the fact that the evidence of abnormally high serum concentrations of PRL has been proven to be associated with poor prognosis in breast and prostate cancer patients. Moreover, since hyperprolactinemia negatively influences the efficacy of anticancer therapies in breast cancer, it could be fundamental to achieve a normalization of PRL levels by long-acting dopaminergic agents, such as cabergoline. On this basis, a study was planned to evaluate the effect of cabergoline on PRL levels in hyperprolactinemic metastatic breast cancer subjects. The study included 20 hyperprolactinemic metastatic breast cancer subjects, who were randomized to receive no therapy or cabergoline at 0.5 mg/week orally for 4 consecutive weeks. Cabergoline therapy induced a normalization in all patients, whereas no spontaneous normalization of PRL levels occured in the control group. These results show that a weekly oral administration of the long-acting dopaminergic agent cabergoline is a well tolerated and effective treatment of metastatic breast cancer-related hyperprolactinemia. The possible prognostic impact of PRL normalization needs to be established by successive studies.