Cytogenetic analysis was performed on megakaryocyte-enriched cell fractions obtained by separation on discontinuous Percoll gradients from blood, bone marrow, and ascitic fluid of 12 patients with myelofibrosis with myeloid metaplasia (MMM). The most common abnormalities appeared to involve chromosomes 1, 3, 7, 8, 11, 13, 21 and 22. Although no specific chromosome rearrangements were observed in megakaryocytic fractions from different tissues of the patients with MMM, the presence in one case of a 3q-abnormality restricted to polyploid metaphases only, and of a 3q paracentric inversion in another patient, seems to confirm a close relationship between the involvement of chromosome 3 and dysplastic thrombopoiesis