Two rare cases of Burkitt's lymphoma presenting as acute leukemia are described. Both patients had typical features of Burkitt's lymphoma with rapidly growing, extranodal tumor masses which were multifocal. The blasts infiltrating the bone marrow and peripheral blood showed cytologic, cytochemical, immunologic, and ultrastructural features of Burkitt's tumor cells. The cells had B-cell markers but lacked the EBV-DNA genome and had multiple microvilli as seen by means of scanning electron microscopy. In both cases, the disease was resistant to chemotherapy and rapidly fatal, despite the fact that complete remissions of short duration was obtained.