Four cases of T cell neoplasia are reported: three presenting as T cell acute lymphoblastic leukemia and one presenting in the leukemic phase of a T cell lymphoma. In all cases, the cells of the leukemic clone were characterized by an identical cytogenetic abnormality. This abnormality was a unique reciprocal translocation involving chromosomes 10 and 14. The breakpoint in chromosome 14 was in band q11, coincident with the assigned locus of the alpha-chain gene of the T cell antigen receptor. The breakpoint in chromosome 10 was in band q24, a region reported to include the locus of the terminal deoxynucleotidyltransferase (TdT) gene. Our observations suggest that translocation t(10;14)(q24;q11) is specific for T cell neoplasia and that a gene in chromosomal band 10q24, possibly the TdT gene, plays an important role in T cell neoplasia when its expression or coding sequence is altered by aberrant recombination involving a T cell antigen receptor gene.