The type I growth factor receptor family has been found to play an important role in the control of normal growth and differentiation. Moreover, the epidermal growth factor receptor and the c-erbB-2 oncogene seem to be implicated in the pathogenesis and behaviour of several cancers, including breast cancer. c-erbB-3 is a new member of the type I receptor family for which there is currently little information available on its expression in neoplastic tissues, and on its possible prognostic significance. This study was undertaken to define the prognostic value of c-erbB-3 expression in a series of node-negative breast cancer (NNBC) patients when compared, by multivariate analysis, with expression of the c-erbB-2 protein and conventional clinicopathological features. cerbB-3 was recognised by the novel monoclonal antibody RTJ1, whereas c-erbB-2 was detected by the polyclonal antibody 21N, using immunocytochemical methods. We found that overexpression of c-erbB-3 occurs frequently in NNBC. Overall, 138 of 212 carcinomas (65%) had some degree of membrane RTJ1 staining, and 28 (13%) showed strong and generalised positivity ( ). Twenty-four per cent of carcinomas had membrane 21N staining, and 12% presented strong and generalised positivity ( ). c-erbB-3 protein expression was significantly associated only with that of c-erbB-2 (P = 0.05), whereas 21N positivity was significantly associated with small tumour size (P = 0.02) and ductal histotype (P = 0.04). No significant correlation between expression of either receptor proteins or relapse-free survival was observed after a median follow-up of 63 months. Applying multivariate analysis, only tumour size approached significance. Our results indicate that analysis of expression of c-erbB-3 and c-erbB-2 alone do not seem to be useful in identifying patients with NNBC at different risk of relapse or death.