Cytogenetic examination of bone marrow cells was performed in 43 patients with myelodysplastic syndrome (MDS). MDS was diagnosed from bone marrow biopsies and smears according to the FAB classification. Of all 43 patients 24 (56%) had clonal karyotype changes including frequently monosomies of chromosomes #5 and #7 as well as interstitial deletions of the long arm of #5, 5 q-. Chromosome aberrations were observed in patients belonging to all FAB-subgroups, e.g. 11/15 patients with RA, 6/10 with RAEB, 5/9 with RAEB/T, and 1/9 with CMMoL. Complex chromosome aberrations involving more than 2 chromosomes occurred predominantly in patients with RAEB/T (4/9) but also in patients with RA (2/15) and RAEB (2/10), and correlated significantly (p less than 0.002) with a shorter survival. No correlation was found between chromosome aberrations and the development of ANLL in 9/43 patients (21%).