The authors report on a cytogenetic survey of 61 patients with preleukemic syndrome (PLS). Of these, 41 had a myeloproliferative disease (MPD) and 20 a myelodysplastic syndrome (MDS). Clonal chromosome abnormalities appeared in 24 patients (39.3%) at disease onset. Such changes had a frequency of 26.8% in patients with MPD and 65% in those with MDS. The authors stress the usefulness of ethidium bromide high resolution techniques. They allow obtaining a larger number of metaphases and elongated chromosomes with higher banding resolution and could account for the frequent detection of chromosome changes in most groups of MDS patients in the present series. Moreover, they discuss the possible significance of some chromosome aberrations suggesting that patients with MPD may live longer than those with MDS because of their higher frequency of normal karyotypes.