The prognostic significance of c-erbB-2 oncoprotein expression was studied using the monoclonal antibody, anti-c-erbB-2 (CB-11, BioGenex) and the avidin-biotin-complex (ABC) technique. Four hundred and ninety patients with primary breast carcinoma diagnosed at Gentofte Hospital in the period 1980-1985 were included. Information about treatment, relapse-free period and overall survival was obtained from the Danish Breast Cancer Co-operative Group (DBCG). The mean follow-up period was more than 10 years. Fifteen per cent of the tumours showed positive immunoreactivity for c-erbB-2. Oncoprotein expression was correlated with presence of lymph node metastases, type of tumour, high number of mitoses, severe nuclear pleomorphism, high histological grade (poor differentiation), and absence of steroid hormone receptors. By univariate analysis, expression of c-erbB-2 oncoprotein, was correlated with poorer overall survival and shorter disease-free period in the entire cohort and in patients with lymph metastases, but not in the group of patients without lymph node metastases. By multivariate analysis, c-erbB-2 failed to be an independent prognostic marker for either disease-free period or overall survival, whereas classical histopathological parameters such as presence of lymph node metastases, high number of mitoses, high histological grade (poor differentiation) and absence of progesterone receptors turned out to be of independent prognostic significance.