Flow cytometric analysis of DNA ploidy was performed on prostatic adenocarcinoma specimens from 80 patients. In all these patients a radical retropubic prostatectomy had been performed. The nuclei for DNA ploidy determination were extracted from paraffin-embedded material of whole sections of the prostate from patients treated by radical prostatectomy between 1980 and 1985. DNA ploidy was a strong prognostic indicator independent of tumor grade and tumor stage. DNA ploidy offered additional information on both tumor stage and tumor grade. In stage C disease the likelihood of progression-free survival was 89.5% in diploid tumors and 27.8% in aneuploid tumors after 9 years. In tetraploid tumors all patients progressed after 9 years. The computed survival rates in stage C disease showed that patients with diploid tumors did significantly better than those with aneuploid or tetraploid tumor patterns. These data indicate therefore that DNA ploidy patterns determined by flow cytometric analysis provide important additional prognostic information on prostatic adenocarcinoma treated by radical prostatectomy.