ras p21 expression, as indicated by the monoclonal antibody ras 11, was estimated using immunohistochemistry on 69 primary colorectal adenocarcinomas. Also, DNA ploidy and S-phase fraction (SPF) were analysed with flow cytometry. Positive staining for ras 11 tended to be more common in DNA non-diploid tumours (P = 0.11), but was significantly correlated with high SPF (P = 0.038). Positive ras 11 staining, Dukes' stage, DNA ploidy and SPF were related to the recurrence-free interval of patients with Dukes' A-C tumours (P = 0.0014, P = 0.023, P = 0.035 and P = 0.040, respectively). ras 11 staining was a prognostic factor independent of both Dukes' stage and DNA ploidy (P = 0.011). The results indicate that pan ras p21 expression is associated with proliferative activity and has an independent prognostic value in colorectal adenocarcinoma.