The nature of renal anemia has been obscure despite numerous investigations initiated by Bright (1836). The author conducted a statistical analysis, with purpose of elucidating the cause of renal anemia, on peripheral blood pictures and myelograms of a total of 136 patients (80 cases of acute and chronic glomerulonephritis, 35 cases of uremia, 14 cases of nephrosis and 7 cases of polycystic kidneys) being admitted to our department in the past 10 years, and obtained the following results. 1) The hemoglobin and RBC showed a inverse correlation with the NPN and the color index tended to be normochromic or hyperchromic in the presence of elevated NPN. 2) The leukocyte count, as a rule, was within normal limits except for leukopenia observed in severe and prolonged azotemia. 3) The absolute lymphocyte count in the peripheral blood was decreased in azotemia. 4) The platelet count had a inverse correlation with the NPN. 5) Nucleated cells in the bone marrow were decreased in number in severe azotemic cases. 6) Myelograms in azotemia revealed marked maturation arrest and depression of the erythroid series. 7) There was normal to slightly deceased cellularity of the myeloid series in azotemia. 8) The serum iron tended to show a proportional correlation with the NPN and a inverse correlation with the RBC. 9) The serum copper became elevated prior to the increase of the serum iron. 10) In azotemia many sideroblasts appeared in the bone marrow, the sideroblast ratio being over 1.0. 11) Sideroblastograms assumed the normal pattern in nephrosis and the hypoplastic pattern in uremia. In chronic glomerulonephritis there was the intermediate pattern of the two. From the results obtained, it is most likely that renal anemia results from the deleterious effects upon bone marrow hematopoiesis, particularly upon erythropoiesis of the noxious substances present in azotemia. Such a bone marrow dysfunction of severe degree and of prolonged duration would ultimately lead to the development of a clinical picture compatible with hypoplastic anemia.