Bleeding tendency in acute leukemia is usually ascribed to thrombocytopen However, it has also been attributed to vascular and capillary defects,1-3 fibrinolysis,4-7 prothrombin complex deficiencies,8,9 and platelet function abnormalities.10-15 Bonnin13,14 has emphasized the thromboplastic function of platelets in determining the magnitude of bleeding and response to treatment in thrombocytopenia. Gaydos et al16 have again emphasized the importance of thrombocytopenia in the incidence and severity of bleeding in acute leukemia. We have been impressed with the number of patients with acute leukemia who have moderate-to-severe bleeding notwithstanding an adequate number of platelets, but who have a platelet function defect. Of particular interest are patients who have similar qualitative platelet abnormalities, but in whom the diagnosis of leukemia cannot be made with certainty at the time that the bleeding diathesis is discovered, although subsequently acute leukemia becomes typically manifest. This report delineates the data for 13