Loss of a chromosome 7 is associated with a poor prognosis in acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS). Some studies have shown higher frequencies of monosomy 7 (-7) in dividing than nondividing myeloid cells, which might indicate that -7 confers a proliferative advantage on the host cell. As other groups have not been able to confirm this, we compared the -7 frequencies in bone marrow metaphases as studied with conventional cytogenetics and in interphase cells using primed in situ (PRINS) labeling. We found significantly higher -7 frequencies in metaphase than in interphase cells irrespective of diagnosis and presence or absence of additional chromosome aberrations. Further, we found a significant correlation between the -7 percentages in resting and dividing cells. Finally, as our material showed a clear male preponderance, Mitelman’s Catalog of Chromosome Aberrations in Cancer was searched for -7. Of 815 cases with AML or MDS, 491 (60.3%) were found to be men. To our knowledge, this is the first observation of a clear deviation from the 1:1 sex ratio in -7 patients.