The role of circulating tumour markers in providing prognostic information has been scarcely studied. We evaluated the prognostic significance of two mucinous markers: CA 15-3 and CA 125 in 115 breast cancer patients at first recurrence of disease. At diagnosis of advanced disease bone involvement was found in 64 patients, lung in 57, skin lymph nodes in 21, liver in 20, and brain in 5. Patients were recruited and treated in the same institution with conventional chemo- or endocrine therapy. The follow-up ranged from 3 to 54+ months (median 35). Serum samples were drawn at first recurrence of disease before the start of any endocrine and/or chemotherapy. Patients with CA 15-3 < 30 U/ml survived significantly longer than those with CA 15-3 > 30 U/ml (median 50+ versus 26 months, P < 0.02). Similarly, overall survival of patients with CA 125 < 35 U/ml was significantly higher in comparison with patients with CA 125 > 35 U/ml (median 34.5 versus 18.5 months, P < 0.001). CA 125, but not CA 15-3, maintained its prognostic value in the subgroup of patients with visceral metastases. Both markers were found to be independent prognostic variables in multivariate analysis according to Cox's model. CA 15-3 and CA 125 appeared to be powerful prognostic indicators, in addition to visceral metastases, in patients with advanced breast cancer.