Abnormalities of the EGF receptor and/or the related ERBB2 receptor occur in a significant proportion of cases of human breast cancer and are important influences in the behaviour of this tumour type. In this report we demonstrate by nucleic acid analysis and immunohistochemistry that the recently recognised third member of this gene family, ERBB3, shows a wide range of expression in breast cancer, and shows stronger immunoreactivity than that observed in normal tissue in 43 out of 195 cases (22%) of primary breast cancer. Overexpression of ERBB3 appears to result from increased levels of gene transcription since in none of the cell lines or primary cancers analysed did we find evidence of gene amplification. High expression of ERBB3 is positively associated with the presence of lymph node metastases, but there was no demonstrable relationship with patient survival in this series.