The mechanism by which angiotensin-converting enzyme inhibitors reduce red cell mass in renal transplant recipients with erythrocytosis is unclear. To examine the role of angiotensin II in this disorder, losartan (a competitive antagonist of the angiotensin II type 1 [AT1] receptor) was administered to 23 patients with erythrocytosis. Fourteen patients took 25 mg/d for 8 wk; nine others were treated with 50 mg/d for 8 wk. Hematocrit decreased from 0.527 ± 0.027 to 0.487 ± 0.045 after 8 wk (P < 0.01)-by at least 0.04 in 19 patients. Decrement in hematocrit in the initial 8 wk of therapy was significantly greater in patients administered 50 mg/d than in patients on 25 mg/d. Twelve of 14 patients initially treated with 25 mg/d showed a small change in hematocrit; the dose was increased to 50 mg/d for 8 more wk. Hematocrit decreased from 0.528 ± 0.030 before losarran treatment to 0.483 ± 0.055 after 16 wk (P < 0.01). After therapy, serum erythropoietin significantly decreased in eight patients with elevated baseline levels, but not in 15 patients with normal baseline levels; however, hematocrit significantly decreased in both groups. Losman was withdrawn in 16 patients; hematocrit increased from 0.440 ± 0.057 to 0.495 ± 0.049 after 8.9 ± 7.5 wk (P < 0.001), without change in serum erythropoietin. Thus, specific blockade of AT receptors inhibited erythropoiesis, suggesting a pathogenic role for angiotensin II in posttransplant erythrocytosis.