To investigate the clinical significance of an immune response to the MUC-1 encoded polymorphic epithelial mucin (PEM) breast cancer, circulating immune complexes containing PEM (PEM.CIC) were measured in sera from 96 healthy women, in pretreatment serum samples from 40 patients with benign breast tumours and from 140 patients with breast cancer and in serum samples from 61 breast cancer patients with recurrent or progressive disease. PEM.CIC were measured using a sandwich enzyme-linked immunoassay, and PEM serum levels were measured with CA 15.3 IRMA (Centocor Inc., Malvern, Pennsylvania, U.S.A.). Cut-off levels used for PEM.CIC and CA 15.3 were 120 Optical Density Units (O.D.) x 10(3) and 30 U/ml, respectively. In benign tumours, positivity for PEM.CIC was 37.5% (15/40). 36 of the 140 patients (25.7%) in the breast cancer pretreatment group had elevated PEM.CIC values. In patients with advanced metastatic disease, positivity for PEM.CIC was 18% (11/61). PEM.CIC was elevated in 32% (24/74) of node-negative patients, but only in 20% (12/59) of node-positive patients and absolute values were higher in node-negative patients (Mann-Whitney U test, two-tailed P = 0.0168). There was an inverse correlation between positivity for PEM.CIC and extent of disease: while 3 of the 6 patients with a carcinoma in situ were positive, only 1 of the 15 patients with more than five nodes involved had elevated levels of PEM.CIC. All 7 patients with distant metastases at first diagnosis were PEM.CIC negative. 28 out of 133 patients had a recurrence during the observation period (median 55 months, range 27-84 months). 23 of these 28 patients (82%) were PEM.CIC negative at the moment of first diagnosis. None of the patients with pretreatment elevation of both PEM.CIC and CA 15.3 (n = 13) relapsed. Our preliminary clinical results suggest that a humoral immune response to PEM protects against disease progression, and further support the idea of using synthetic peptides or glycopeptides containing the immunogenic core of the mucin as cancer vaccines.