We characterized the T-cell receptor (TCR) gene rearrangements and sequences in 15 T-lineage acute lymphoblastic leukemia (T-ALL) and seven adult T-cell leukemia (ATL) samples. Southern blot analysis showed that neither of the two TCR delta alleles was deleted in two T-ALL samples, suggesting that the TCR alpha loci have a germ line configuration. The TCR alpha and beta sequences were cloned and sequenced by reverse transcriptase-inverse polymerase chain reaction. Two T-ALL samples had a long complementarity determining region (CDR), three of the alpha chain and the other two T-ALL samples had long CDR3 of the beta chain, compared with normal peripheral blood lymphocytes (PBL). Thus, a total of six T-ALL samples had unusual TCR gene structure, which was unrelated to the immunophenotype. On the other hand, CDR3 length in ATL samples was similar to normal PBL. These data suggest that T-ALL is derived from an immature T-cell repertoire which undergoes TCR gene rearrangement or has not been negatively selected.