Polycythemia is consistently produced in animals undergoing experimental hydronephrosis. This hematologic response is thought to be mediated by a microcirculatory insult and a more accelerated elaboration of erythropoietin by the hydronephrotic kidney. While an association between hydronephrosis and polycythemia has been reported in humans, it is a relatively rare event. Further studies are necessary to explain why this should be so, as well as to confirm that the polycythemia in humans is secondary to an increased production of erythropoietin.