Myeloma is essentially a malignant tumor of the bone marrow, which may present itself in single or multiple foci and tends to involve notably the ribs, spine, skull, and pelvis. The individual tumors develop from the primitive cells of the hemopoietic system and have been accorded a variety of names, as erythroblastoma, plasmocytoma, myelocytoma, and lymphocytoma. In the later stages of the disease, myelomatous deposits may appear in the liver, spleen, and lymph nodes—rarely in the lungs and other viscera (12). It was our clinical impression that these lesions were not particularly radiosensitive. Therefore, it was with some interest and surprise that we recently noted statements from two authorities to the effect that myeloma was regarded as a very radiosensitive lesion. Shields Warren (30) observed in February 1947 that “multiple myeloma is a radiosensitive tumor—to be classified with lymphoma and Ewing's tumor of bone!” In the same year del Regato (1) wrote: “Myelomas are very radiosensitive and locally radiocurable.” In view of these observations, we decided to re-examine the recent literature and to review the cases of multiple myeloma which we had been privileged to see in the radiation therapy department at the San Francisco Hospital during the last two decades. Incidence Multiple myeloma is a rare disease. Batts (3) reported 40 cases in a survey of almost 300,000 admissions to the University of Michigan Hospital. Bayrd and Heck (4) reported 83 proved cases out of a total of 350 with a clinical diagnosis of multiple myeloma, obtained from all patients admitted to the Mayo Clinic up to the year 1946. Ehrlich (10), in a ten-year period at the New York Cancer Institute, saw 7 cases among some 127 primary bone tumors; only 3 of these were verified microscopically. The sexes are involved in a ratio of approximately 1.5 males to 1 female. The incidence is highest between the ages of forty and seventy years, but the disease has been reported in persons as young as sixteen months and as old as seventy-eight years. Pathology Myelomatous tumors are usually gelatinous in appearance and from red to gray in color. Codman (8) in 1925 subdivided them into lymphocytic, myelocytic, erythroblastic, and plasma-cell types. Longcope (18) in 1927 added a fifth, the myeloblastic. The plasmocytomas are the most frequent and the erythroblastomas probably the least so. Wintrobe (31) has pointed out that the plasma cells of the myelomas differ in certain respects from ordinary plasma cells. The chromatin of the nuclei of the cells of the plasmocytic myelomas does not show the characteristic “spoke-wheel” arrangement, and the perinuclear clear areas seen in true plasma cells are hardly ever observed. The tumor cells have no osteoclastic or osteoblastic properties. They form small nodules which grow by expansion and tend to necrosis and hemorrhage.