In the present study the prognostic value of tumor S-phase fraction was evaluated in 69 patients with renal-cell carcinoma. The proportion of S-phase cells was calculated from DNA histograms by flow cytometry in multiple samples from each tumor. The mean tumor S-phase percentage varied between 1.0 and 17.0%, mean 7.5%, with a significant difference between diploid and non-diploid tumors. Stage-I tumors showed significantly lower S-phase values than tumors of stages 11/111 and IV. Histopathological grade correlated with fraction of cells in S-phase. Nineteen tumors were homogeneously diploid, I was tetraploid and 49 were aneuploid. Heterogeneity concerning S-phase values was found in 46 of the 69 tumors and concerning DNA ploidy in 34 tumors. Survival time was longer for patients with diploid tumors than for those with aneuploid tumors. Patients with S-phase values <7.5% had a longer survival time than patients having tumors with S-phase values >7.5%. Within the group of aneuploid tumors, patients with S-phase values <7.5% had a significantly better prognosis. In multivariate analysis, only tumor stage and S-phase gave significant independent prognostic information. The S-phase fraction seems to be an additional prognostic parameter for patients with renal-cell carcinoma.