A cytogenetic study was performed in five patients with Sézary syndrome. Metaphases were obtained from a phytohemagglutinin-stimulated lymphocyte culture. The five patients showed abnormal karyotypes. The chromosomes preferentially involved in numerical aberrations were chromosomes 10 (monosomy) and 13 (monosomy); involved in structural changes were chromosomes 1, 2, 4, 6, and 14. In our series, all patients showed progression of the disease.