Delayed hemolytic reaction is a well recognized hazard of blood transfusion and occurs mainly in recipients with alloantibodies, due to sensitization to red cell antigens by previous transfusion or pregnancy. Less frequently, such a reaction may be associated with the presence of red cell autoantibodies appearing after alloimmunization. We report the observation of a patient with chronic myelomonocytic leukemia, who presented a transitory episode of delayed hemolysis after multiple transfusions. At first, anti-Kell alloantibodies were identified and, two weeks later, warm "broad-specific" autoantibodies were detected by direct and indirect antiglobulin tests. The anemia was associated with unstable angina. The patient received multiple transfusions and prednisone. After 3 weeks, the transfusion needs diminished and the autoantibodies progressively disappeared from the serum. The direct antiglobulin test was negative 4 months later. This observation illustrates the poorly understood relationships between erythrocytic alloantigen exposure and red cell autoimmunization.