THE MATERIAL for this article was collected over the years 1930 through 1953 at the Los Angeles Tumor Institute (Drs. Costolow, Meland, Johnson, Nolan, and Hare), and patients were referred by urologists and surgeons for postoperative radiation therapy. Seventy-three patients with retroperitoneal tumors were accepted for treatment. In some instances, the postoperative treatment was given immediately after surgery, and in others it was commenced only after recurrences or metastases had occurred. This caused a wide variation in the stage of disease, the tumor dosage, and the survival rate. With such diversity of cases, it is impossible to obtain survival percentages of statistical significance. It is our belief, however, that demonstrable regression of tumor and increased survival time after postoperative irradiation, or irradiation alone in the case of inoperable tumors, do demonstrate the value of these modalities. The widely varied origin and histology of these tumors make prognosis difficult. In many