Rearrangements affecting the long arm of chromosome 3 have been linked to abnormalities of platelet production and morphology in hematologic disorders. We present five patients with myelodysplastic syndromes or acute leukemia in whom inv(3)(q21q26) was seen. One of the patients showed an inversion of both homologs involving precisely the same breakpoints, suggesting apparent duplication of the abnormal chromosome 3 and loss of the normal homolog. To our knowledge this is the first report detailing such a condition.