Sialyl-Tn (STn) is a carcinoma-associated carbohydrate determinant expressed on cancer-associated mucins and has the structure NANA alpha(2-6)alpha GalNAc. Expression of STn in colon and ovarian cancer is associated with a poor prognosis independent of tumour grade, stage or histological type. We have examined 237 cases of primary breast cancer for expression of this antigen using the antibody HB-STn (Dako). The frequency of STn expression was 31% in the whole group, 36% in the node-negative and 28% in the node-positive group. Survival was lower, but not significantly so, in the STn-positive group (P = 0.07), but this effect was highly significant for patients with node-positive disease (P < 0.002), the curves for node-negative disease being coincident (P = 0.31). In node-positive disease the effect was limited to those receiving adjuvant chemotherapy (P = 0.001). In a multivariate (Cox) analysis on the whole group STn staining, combined with adjuvant chemotherapy, showed a highly significant correlation with survival. In STn-negative cases, adjuvant chemotherapy improved survival (relative risk 2.3, 95% confidence intervals 1.4-3.9), whereas adjuvant chemotherapy did not influence survival in patients which expressed STn (relative risk 1.1, 95% confidence intervals 0.6-2.2). Thus, by either direct or indirect mechanisms, STn positivity appears to be a marker of resistance to adjuvant chemotherapy.