Over a 16-year period (1966 to 1981), 349 patients underwent radical retropubic prostatectomy for pathologic stage B adenocarcinoma of the prostate. Nuclear DNA content was measured by flow cytometry on available archival material of 283 patients. Two hundred sixty-one patients (92%) had high-quality histograms. The ploidy distribution was as follows: DNA diploid, 177 (68%); DNA tetraploid, 74 (28%); and DNA aneuploid, 10 (4%). The average follow-up was 9.4 years. At the time of follow-up, 53 patients (20%) within the study group had developed tumor progression: 22 local, 23 systemic, and 8 both. The ploidy distribution of the population that developed tumor progression was 27 DNA diploid (51%), 16 DNA tetraploid (30%), and 10 DNA aneuploid (19%). This ploidy distribution is significantly different from that found for the nonprogression group with stage B disease. Overall, 31% of patients with DNA nondiploid tumors had tumors that progressed compared with 15% of patients with DNA diploid tumors. All (100%) DNA aneuploid tumors progressed. The DNA ploidy distribution of all pathologic stage B prostate cancers differs significantly from that found in more advanced stages (C and D1) previously reported for the same time interval. However, the ploidy distribution of stage B tumors that progressed closely resembles that of the stage C and D1 tumors. These results further support the working hypothesis that nuclear DNA content has marked prognostic significance for patients with adenocarcinoma of the prostate. It seems to us that analysis of ploidy by flow or static cytometry will become an essential tool for treating patients with localized prostate cancer.