Immunosuppression of recipients of renal transplants with azathioprine has been associated with two major side effects: hepatotoxicity and myelotoxicity, mainly in the form of leukopenia. Reports of isolated anemia in these patients have been rare. We now observed the development of severe anemia in 9 out of 11 renal transplant recipients whose immunosuppressive regimen was converted from cyclosporine plus prednisone to azathioprine plus prednisone. A significant (P = 0.001) drop in hematocrit (from 34 +/- 4% to 27 +/- 3%, mean +/- SD) and hemoglobin (from 11.6 +/- 1.3 g/dl to 9.5 +/- 1.0 g/dl) was found. Since a common variable of all these patients was their use of an angiotensin-converting enzyme (ACE) inhibitor as antihypertensive medication, we speculated that the combination of azathioprine and ACE blocker might be the reason for the anemia. We then compared 2 groups of 10 patients each who had been on azathioprine as their regular immunosuppressive agent and who did or did not take an ACE inhibitor. Hematocrit and hemoglobin were significantly (P = 0.01) lower in the group of patients taking ACE inhibitors (33 +/- 6% versus 41 +/- 5% and 11.5 +/- 2.0 g/dl versus 14.0 +/- 1.6 g/dl, respectively). Haptoglobin levels were also significantly (P = 0.05) lower in the ACE inhibitor group (116 +/- 65 mg/dl versus 210 +/- 114 mg/dl). Erythropoietin concentration in the serum and the reticulocyte index were slightly, but not significantly, higher in the ACE inhibitor group but the values were probably too low for their degree of anemia. Comparing hematological parameters of the patients in the ACE inhibitor group before and after beginning of the antihypertensive treatment confirmed a significant reduction of hematocrit and hemoglobin following therapy with an ACE inhibitor. Hematocrit fell from 41 +/- 7% to 36 +/- 6% and hemoglobin from 14.0 +/- 2.3 g/dl to 11.3 +/- 1.5 g/dl (P < 0.05 for both). We conclude that the combination of these two drugs should probably be avoided.