The aim of this study was to determine the relative rate of c-erbB-2 oncoprotein immunodetection on matched fine needle aspiration (FNA) smears and surgical specimens of breast cancer, and to correlate the c-erbB-2 expression with the assessment of the DNA ploidy status. The expression of c-erbB-2 oncoprotein was evaluated using an immunoalkaline phosphatase technique in 49 breast aspirates (four benign and 45 malignant lesions) and 21 matched surgical specimens. The DNA ploidy status was assessed by densitometric techniques on Feulgen-stained smears. Fifty-eight per cent of the smears obtained from 45 malignant lesions and 43% of the 21 corresponding paraffin sections contained cells that were stained by the antibody. The higher incidence of c-erbB-2 expression on smears seems to be due mainly to the better antigen preservation in the fresh cytological preparations. The correlation between c-erbB-2 oncogene expression and DNA ploidy assessment showed an increased incidence of oncogene expression in aneuploid tumours (71% vs 29%; P < 0.05).