There is, as yet, no tumour marker which is sufficiently specific and sensitive for use in the routine assessment of breast cancer patients. CA15.3 is a recently described tumour marker determined by two monoclonal antibodies. We have estimated CA15.3 by immunoradiometric assay in 187 patients attending a breast clinic. Eighty-one patients with benign disease were used as controls and 32 U/ml was taken as the upper limit of the normal range (means + 3SD = 31.7). Of 58 women with Stage I and II disease, only four had abnormal concentrations of CA15.3 and all are disease-free at a mean follow-up of 31 months. Seven women with normal CA15.3 concentrations developed recurrent disease at a mean of 18.7 months (range 10-25 months). Seven-day postoperative values were significantly lower than pre-operative values. There was no association between the CA15.3 value and the axillary nodal status. The patients with disseminated disease had a wide range of CA15.3 concentrations and there was no association between the CA15.3 concentration and the apparent tumour load.