Bearing in mind the continuing controversy over the prognostic significance of epidermal growth factor receptor (EGF-r) expression, we investigated its clinical significance prospectively in 345 primary breast cancer patients. The prognostic significance of EGF-r expression, as measured by a radioligand binding assay, was determined by Cox's multivariate analysis using EGF-r concentration as a continuous or dichotomous variable. Increased EGF-r expression was detected in 2632% of tumours, depending on the cut-off in concentration used. EGF-r expression, irrespective of the cut-off, was not associated with tumour size or grade or the number of axillary nodes involved. There was, however, a strong inverse association between EGF-r expression and the absence of hormone receptors. After a median follow-up period of 57 months, multivariate analysis suggested that EGF-r expression was associated with increases in risk for both relapse and death from breast cancer, even after adjusting for oestrogen receptor (ER) concentration, tumour size and the number of axillary nodes involved. Patients with ER-positive tumours, which also expressed EGF-r, had increases in risk for both relapse and death from breast cancer compared with tumours without EGF-r. Expression of EGF-r was not a predictor of poor prognosis in either node-negative or ERnegative subgroups of patients.