Overexpression of the c-erbB-2 oncogene has been demonstrated in a variety of tumours, including colorectal tumours. In breast carcinoma, c-erbB-2 overexpression is associated with DNA ploidy, some other prognostic indicators, and unfavourable survival prospects. However, there is little such information available regarding colorectal tumours. In this study, c-erbB-2 was analysed retrospectively by immunohistochemistry in 293 primary colorectal adenocarcinomas to assess its relation to DNA ploidy, S-phase fraction, other prognostic factors, and patient survival. Using the monoclonal antibody NCL-CB11, we found that 23% of the tumours were strongly c-erbB-2 positive, while 36% showed weak expression. The highest frequency of c-erbB-2 expression was 81% in DNA tetraploid tumours, compared to 63% in aneuploid and, 53% in diploid tumours (test for heterogeneity, p = 0.031). Overexpression of c-erbB-2 indicated a favourable prognosis in patients with DNA aneuploid tumours (p = 0.0088), but not in those with diploid or tetraploid tumours. The prognostic value of c-erbB-2 in DNA aneuploid tumours remained even after adjustment for Dukes' stage (p = 0.027). The results suggest that a combination of c-erbB-2 expression and DNA ploidy may improve the identification of patients' risk of cancer death.