An immunohistochemical study was performed on 211 primary breast carcinomas for c-erbB-2 expression. All patients had involvement of axillary lymph nodes and all were randomised onto one of the Ludwig Breast Cancer Trials I-IV between July 1978 and August 1981. c-erbB-2 overexpression significantly correlated with high S-phase fraction, four or more positive axillary nodes involved, estrogen receptor negative primaries, progesterone receptor negative primaries, high grade tumours and DNA aneuploidy. With a nine year median follow-up c-erbB-2 positive tumours had worse disease-free survival (p = 0.0002) and overall survival (p less than 0.0001). Multivariate analyses using proportional hazard regression models demonstrated that c-erbB-2 positivity continued to predict a poor outcome even when accounting for the effects of other prognostic factors.