The expression of Rb protein was analysed by immunocytochemical methods in 118 patients with T12M0 prostatic adenocarcinoma who were followed-up for more than 12 years. Rb protein was expressed in 117/118 (99%) tumours. The fraction of positive nuclei was (mean +/- SD, 92 +/- 19%) and only one tumour was completely negative for Rb protein. Abnormal expression of the Rb protein (in fewer than 90% of cells) was independent of the T category while there was a significant relationship between DNA ploidy (P = 0.0138), S-phase fraction (P = 0.0042) and expression of the Rb protein. Abnormal expression of the Rb protein had no prognostic value while T category (P = 0.0017), Gleason score (P = 0.0097), DNA ploidy (P = 0.0034), S-phase fraction (P = 0.0179) and mitotic index (P = 0.0001) were significant prognostic factors. In multivariate analysis the only independent predictor was the mitotic index [Risk ratio (RR) (95%CI) = 7.4(2.4-22.5), P = 0.004]. The results show that Rb protein immunohistochemistry has hardly any prognostic significance in prostatic adenocarcinoma whereas direct measurement of cell proliferative activity contains significant prognostic information.