Two renal transplant recipients with reversible bone marrow damage in the course of a simultaneous azathioprine-allopurinol therapy are discussed. Anemia, leukocytopenia and thrombocytopenia were found in both patients. Discontinuation of the azathioprine-allopurinol treatment was followed by increase of hematocrit, hemoglobin, erythrocytes, white blood cells and platelets. Interaction of azathioprine and allopurinol seems to be responsible for bone marrow damage in these patients. It can be concluded that the dose of azathioprine should be reduced when allopurinol is given concomitantly.