Ploidy level determinations of prostatic carcinoma in 213 patients selected on the basis of survival time were analyzed. Two extreme groups were chosen: extreme group 1, 131 patients who died from prostatic cancer within 3 years of diagnosis; and extreme group 2, 82 patients who survived for more than 15 years after diagnosis. All patients were diagnosed by fine needle aspiration biopsy. The DNA measurements were performed on Feulgen stained, destained May-Grünwald-Giemsa smears. The DNA distribution patterns were studied in three benign prostatic lesions as a base for analyses of prostatic carcinoma. By choosing two extreme groups it was possible to evaluate in detail the optimal limits for defining the diploid 2c region and the tetraploid 4c region. Various limits were tested in order to determine those which most clearly separate extreme group 1 from extreme group 2. We found that the optimal upper diploid limit was 2.5c and the optimal tetraploid limits were 3.5c-4.5c. By using these limits to determine the percentage of aberrant tumor cells, i.e., non-2c and non-4c, and combining this with the modal value (in c units) of the tumor cell population the tumors could unambiguously be divided into D-, T-, and A-types. D/T-type tumors were found only in extreme group 2 and had less than 30% aberrant tumor cells, while A-type tumors (high-grade aneuploid) had greater than 50% non-2c and non-4c tumor cells. All A-type tumors were found in extreme group 1. In order to investigate whether the classification of tumors into D-, T-, and A-type was valid in general and could also be applied to patients with survival time between the two extreme groups (3-15 years), a material of 79 patients with a wide range of survival times was tested. The tumors were classified according to the above-mentioned criteria into A-, D-, and T-type tumors. All patients who died within 5 years of diagnosis had A-type tumors. All patients who lived greater than 5 years from diagnosis had D- or T-type tumors.