The cytogenetic studies performed on 7 patients diagnosed of Sezary's syndrome are reported. The chromosomal study was made after 72 hours of culture of phytohaemagglutinin-stimulated peripheral blood. The 7 patients had abnormal karyotypes, the numeral alterations involving chromosomes 10 and 13, whereas the structural abnormalities affected chromosomes 1, 2, 4, 6 and 14. The large-cell variant has been associated with tetraploidy and the small-cell variant with diploidy, but this fact was not confirmed in the present series.