The development of erythrocytosis following renal transplantation has been reported to be caused by a number of factors. These include acute and chronic rejection, hydronephrosis and renal artery stenosis. In this study, seven patients were noted to have erythrocytosis with hematocrits ranging between 53.5 and 66%. Serum erythropoietin levels were elevated and ranged between 11 and 60 mU/ml with a mean of 31.9 mU/ml in six of seven patients. Selective catheterization of veins of native and transplanted kidneys in three patients revealed mean serum levels of 40.9 and 13.0 mU/ml, respectively. This suggests that excess erythropoietin is being produced from the diseased native kidneys. Bilateral nephrectomy in one patient cured erythrocytosis and dropped systemic levels of erythropoietin (EP) to 6.1 mU/ml. In four of the remaining five patients, hematocrits came down spontaneously to within normal over a 1- to 3- year period Consequently, it appears that in a number of transplant patients the retained diseased kidneys, having lost all excretory and concentrating function, may remain capable of functioning as endocrine erythropoietin-producing organs Patients with end stage renal disease on chronic hemodialysis almost invariably suffer a moderate to severe degree of anemia, the cause of which is rather complex and includes decreased red blood cell production and decreased red blood cell survival (1) Shortly after kidney transplantation, however, and in the presence of good to excellent kidney function, most if not all transplant patients overcome their anemic state; their hematocrits once again begin to build up to become normal or close to normal. In a number of instances, however, the development of erythrocytosis has been reported following transplantation. Contributing factors for this development include acute and chronic rejection (2), renal artery stenosis (3), hydronephrosis (4) and, on a few occasions, overproduction of erythropoietin by the native diseased kidneys (5) The purpose of this study is to report the occurrence of erythrocytosis in seven patients with good functioning kidney transplants, to determine the cause of erythrocytosis, and to discern the role and site of erythropoietin production in these patients