The diagnosis of acute megakaryoblastic leukemia (AMKL) (M7, French-American-British [FAB] classification) has rarely been made in children due, in part, to its pleomorphic morphology and ability to mimic other malignancies common in the pediatric age group. Three infants are described who had thrombocytopenia and the classical criteria of metastatic solid tumor in the bone marrow: patchy infiltration by cohesive clusters of cells with high nuclear cytoplasmic ratio and myelofibrosis in the bone marrow biopsy infiltrated area. This finding prompted clinical evaluation for solid tumor. The megakaryocytic lineage was ascertained by immunocytochemical studies and/or electron microscopic examinations of the bone marrow aspirates. The blasts in all three patients showed cytogenetic abnormalities that also were demonstrated by quantitative DNA analysis. None of the infants had Down's syndrome. Two of the patients are alive; one is off of therapy and the other is in remission. The third patient was transferred to another institution and lost to follow-up. Two children had wheezing that disappeared in remission. It is proposed that the clinical symptoms may be due to a substance produced, stored, or released by the leukemic cells.