A case which the authors feel is best classified as acute aleukemic megakaryocyte leukemia is reported. The clinical and pathologic findings are presented, the latter in detail. Reference is made to the controversy about whether this disease should be accepted as a true nosologic entity. The histologic details of the leukemic megakaryocytes are illustrated and the literature is reviewed. The authors believe the case fills a gap in the sequential pathology of the megakaryocytes from increase in numbers of megakaryocytes in cases of agnogenic myeloid metaplasia to examples of megakaryocyte leukemia with massive peripheral thrombocytosis.