Cytogenetic studies were performed on three patients with chronic T cell leukemia and on one patient with T cell lymphoma. One of the patients had leukemic cells derived from a suppressor T cell clone, another expressed OKT3, 4, and 8, and cells of the other two were derived from a helper T cell clone. All patients had an abnormal karyotype in peripheral blood or bone marrow cultured with or without mitogen. Modal chromosome numbers were 42 and 44/45 in one patient each and 47 in the 2 others. The structural and numerical abnormalities involved almost all chromosomes, except no. 19 and the X chromosome. All patients had a rearrangement of the long arm of no. 14, with a break at band 14q11;3 patients also had a break at 14q32. An inversion of 14q occurred in two patients; a tandem translocation involving both no. 14 chromosomes and a translocation between no. 14 and no. 17 each occurred in one patient. The break in 14q at band q11 in our cases resembles the chromosome change reported in ataxia telangiectasia. This provides added support for the proposal that a 14q rearrangement involving band q11–12, with or without an accompanying break in 14q32, may confer a proliferative advantage on lymphocytes, especially on those of T cell origin.