The fourth report of our long-term study concerns 70 cases of primary radiotherapy. The side effects of our radiation technique are minimal and temporary; in over 80% of the cases, potency was not affected. The morphologically demonstrable changes in the tumor tissue, in the original glandular parenchyma, and in the fibromuscular stroma leading to the vessels following high-voltage therapy are described. Three types of progressive patterns, based on the grade of histologic regression, can be determined. In more than half of the cases, with increasing distance from the source of radiation, no tumor or only minimal residual tumor was demonstrable. Sensibility and resistance of carcinoma of the prostate are not dependent upon the grade of differentiation of the tumor. A prognostic statement concerning the success of radiation therapy from initial histologic findings is not possible here. The occurrence of metastases in 18% of the cases following radiation probably resulted from the inexactness of the purely clinical determination of the stage of the tumor. The isolated, local treatment of prostate carcinoma through radiotherapy is justified only after exact determination of the stage of the tumor, i.e., certain exclusion of metastases via a diagnostic lymphadenectomy.