Deoxyribonucleic acid (DNA) flow cytometry and light microscopy were performed in pre-radiotherapy and post-radiotherapy biopsies obtained from the primary tumor in 31 patients with prostate cancer. Radiotherapy was applied by means of transperineal 125iodine (125I) implantation. Of the patients 21 had pretreatment biopsies and in 19 of these biopsies also were performed 1 and/or 1 1/2 years after the 125I implantation. Posttreatment biopsies were available for DNA flow cytometry in 12 additional patients without pretreatment DNA flow cytometry assessment. Of the 21 pretreatment biopsies 7 were diploid, 6 tetraploid and 8 aneuploid. All 31 posttreatment biopsies were either tetraploid (21) or aneuploid (10). All 6 pretreatment diploid tumors became tetraploid after radiotherapy. At 1 and/or 1 1/2 years after 125I implantation residual tumors were found in 28 of 31 prostatic glands. The high frequency of nondiploid DNA stemlines 1 or more years after 125I implantation and the high rate of residual tumor leave some doubt about the radiocurability of prostate cancer by the chosen radiotherapy technique.