To determine if relative nuclear deoxyribonucleic acid (DNA) content is an important prognostic parameter for patients with clinically localized prostate carcinoma treated by external beam radiotherapy, we performed static DNA cytometry on archival paraffin embedded prostate needle biopsy specimens obtained before treatment. DNA content was measured with the Zeiss IBAS 2000 Image Analyzer and the Feulgen staining method. Tumor samples from 65 patients with clinically localized carcinoma of the prostate treated with at least 6,000 cGy. from 1974 to 1980 were studied. Patients and tumors were divided into 2 groups: group 1 - 31 patients with relative DNA content less than 1.5 times normal and group 2 - 34 patients with relative DNA content greater than 1.5 times normal. The prostate cancer nonprogression rate at 10 years was 64% for group 1 and 11% for group 2. Prostate cancer cause specific survival at 10 years was 73% for group 1 and 20% for group 2. These differences are highly significant (p less than 0.0001). By contrast, stratification and analysis according to tumor clinical stage, Mayo histological nuclear grade or Gleason score proved not to be as significant. Cox multivariate analysis also identified DNA content as the most important independent variable for cancer specific survival and progression. Nuclear DNA content measured by static cytometry appears useful in identifying those patients with clinically localized prostate carcinoma who may have a favorable probability of long-term disease control by external beam radiotherapy.