Presence of megakaryocytic cells in patients with myeloid disorders were investigated by staining plastic embedded biopsy sections with an anti-Factor VIII antibody (AFA). Two hundred and fifty cases were studied, 207 of whom had acute myeloid leukemia (AML) while 43 had myelodysplastic syndromes (MDS). Abnormal clusters of AFA positive cells indicating multilineage disease were identified in 17% with primary AML (30/175), 38% with secondary AML (12/32) and 42% cases of MDS (18/43). Biological characteristics of these 60 AFA positive cases were investigated. No unique differences in cell cycle characteristics following bromodeoxyuridine (BrdU) were identified. We confirm several recent reports that the incidence of multilineage involvement in AML is substantial.