The overexpression of the c-erbB-2 oncoprotein is now thought by most authors to be associated with adverse prognosis in breast carcinoma. In this study, we investigate the relationship between overexpression of the c-erbB-2 oncoprotein and nuclear size by morphometry in a series of 150 human breast carcinomas, comprising 65 cases of ductal carcinoma in situ (DCIS) and 85 cases of invasive adenocarcinoma. The mean nuclear size for c-erbB-2 positive cases of DCIS was 54.8 micron 2 and invasive carcinoma was 52.1 micron 2 respectively, in contrast with 41.6 micron 2 and 42.5 micron 2 for c-erbB-2 negative cases of DCIS and invasive carcinoma respectively. Flow cytometric examination of DNA in a subset of 91 of these tumours showed no association between tumour cell aneuploidy and c-erbB-2 overexpression. S-phase fraction could be calculated on 20 cases of DCIS and 48 invasive carcinomas. There was a strong association between c-erbB-2 overexpression, S-phase fraction (p less than 0.001) and proliferative index (p less than 0.001) in 20 cases of DCIS. A weak association of S-phase fraction and c-erbB-2 overexpression was seen in 48 invasive carcinomas (p = 0.047). This study confirms the subjective impression that there is a relationship between large tumour cell nuclear size and an overexpression of the c-erbB-2 oncoprotein, and also shows an association with increased tumour cell proliferation.