Chromosome abnormalities were documented in the PHA-stimulated lymphoid cells from 10 patients with chronic T-cell leukemias (6 T-PLL, 3 T-CLL and 1 T-LCL). One or more marker chromosomes were present in most metaphases in every case of T-PLL but they were different in each case. Structural abnormalities were also observed in T-CLL and T-LCL but less consistently. Chromosomes involved in marker formation in two or more cases were: Nos. 2, 7, 8, 9, 11 and 16. The karyotype was hypodiploid in five cases and hyperdiploid in two. Chromosomes Nos. 2 and 8 were the most frequently involved in structural and numerical abnormalities: Nos. 8 in 2 T-CLL and the 6 T-PLL and No. 2 in 1 T-CLL, 1 T-LCL and 5 T-PLL. Abnormalities of chromosome No. 2 are rare in human neoplastic disease but may be common in T-lymphoproliferative disorders. The differences between T-PLL and T-CLL reported here may have some bearing on the clinical features. T-PLL, characterised in all cases by clonal abnormalities, has an aggressive clinical course. In contrast, T-CLL with less consistent karyotypic changes has a protracted and benign clinical evolution.