Clonal chromosome abnormalities were demonstrated in 13 cases of T cell leukemia with a mature membrane phenotype. The cases comprised two adult T cell lymphoma leukemia (ATLL), five Sézary syndrome (SS), three T prolymphocytic leukemia (T-PLL), and three T chronic lymphocytic leukemia (T-CLL). The presence of clonal chromosome abnormalities in the three cases of T-CLL supports the view that although clinically benign, this disorder probably represents a neoplastic process. Cells from six of 11 patients were tested and found to be reactive with monoclonal antibody (McAb) anti-Tac in the absence of mitogens (two ATLL, two SS, and two T-PLL). Only the two ATLL patients were serologically positive for the human T cell leukemia virus I (HTLV-I), and their cells showed a higher anti-Tac reactivity. Chromosome 7 abnormalities were observed in seven cases (two ATLL, three SS, two T-PLL), and a strong correlation was seen between the presence of a 7q abnormality and the expression of the Tac antigen (T cell growth factor receptor). These findings are discussed in relation to the localization of T cell receptor genes on chromosome 7 and possible mechanisms for the activation of the Tac gene.