The prognostic significance of c-erbB-2 expression was studied in paraffin wax embedded colorectal cancer tissue using a monoclonal antibody. One hundred and sixty-four patients with Dukes' B disease were studied. Membranous staining was not detected in any case. Cytoplasmic c-erbB-2 staining was seen in 55 cancers (33.5%). Cytoplasmic staining was unrelated to patient age (P = 0.31), sex (P = 0.69), tumour site (P = 0.69), size (P = 0.57), histological grade (P = 0.42) or ploidy status (P = 0.21) but was found more frequently in obstructing cancers (P = 0.03). Mean follow up of the patient population was 6.3 years. Five-year-survival estimated by the Kaplan-Meier life-table method was 47% for those with cytoplasmic c-erbB-2 staining and 77% for those without (log rank analysis; P < or = 0.0001). Stepwise regression analysis identified c-erbB-2 staining (relative risk, 2.51; P = 0.0005) and bowel obstruction (relative risk, 1.99; P = 0.015) as independent predictors of survival. It is suggested that cytoplasmic c-erbB-2 expression may provide a useful marker of tumour behaviour in Dukes' B colorectal cancer.