Clonal karyotype abnormalities in 124 patients with myelodysplastic syndrome are reviewed. 36% of patients had abnormalities at referral, the most common being 5q-, +8 and lesions of chromosomes 7 and 20. Reduced survival was associated with the presence of either single or multiple clonal abnormalities at referral, abnormalities of chromosome 7 or 8 (either alone or with other lesions) and exclusively abnormal metaphases. The presence of 5q- alone did not appear to affect survival. Sequential studies were carried out in 77 patients of whom 12 showed karyotypic evolution. Reduced survival was observed in patients with an evolving karyotype but appeared to be due almost entirely to evolution in those patients whose initial karyotype was normal. Leukaemic transformation occurred more commonly in patients with an abnormal karyotype, particularly those with multiple abnormalities, and in patients with an evolving karyotype. Although the first appearance of an abnormal karyotype or an apparent evolution are important phenomena, it is probable that in some cases they merely represent expansion of a previously existing clone that has escaped detection. The distinction between true karyotypic evolution or clonal expansion and statistical variations due to small sample size and variability of samples may be difficult but needs to be taken into account in considering clinical significance.