The prognostic significance of ploidy level was studied in prostatic carcinoma and compared with the prognostic significance of morphologic grade and clinical stage. A nonselected group of 145 patients was studied in whom prostatic carcinoma was diagnosed by fine-needle aspiration biopsy at the Karolinska Hospital in 1966. All patients had endocrine therapy and were observed for 23 years or until death. Ploidy level was determined from cytophotometric measurements of Feulgen-stained tumor cell nuclei. The original May-GrΓΌnwald-Giemsa-stained cytologic slides, on which the cancer diagnoses were based in 1966, were destained and subsequently Feulgen stained for cytophotometric analysis. From the Feulgen-DNA data, the tumors were classified as near-diploid, near-tetraploid, and highly aneuploid, variants D-type, T-type, and A-type, respectively. The A-type tumors progressed rapidly, and 96% of the patients with this type died of the tumor within 5 years and all patients with this type died within 7 years. D-type and T-type tumors progressed much more slowly. None of the patients with these types of tumors died of the tumor disease within the first 5 years after diagnosis. As many as 12% of the patients (crude survival rate; corresponding to a relative survival rate of 43%) were still alive 15 years after diagnosis. According to multivariate Cox regression analysis, ploidy compared with grade and tumor stage was the strongest predictor of survival.