The TAL1 gene is altered as a consequence of t (1; 14)(p32; q11) found in T-cell acute lymphoblastic leukemia (ALL) and shows site specific recombination (tald rearrangement). We investigated TAL1 gene alterations in 39 children with T-cell ALL, in 32 with B-precursor ALL, in three with ALL with myeloid-associated antigen, and in 18 with T-non-Hodgkin's lymphoma (T-NHL). tald rearrangement was found in nine of 39 T-cell ALL patients using Southern blot analysis with a TAL1 gene probe. Polymerase chain reaction (PCR) products predicted from the sequences of the corresponding tald alleles were shown in all of these patients. In contrast, no rearranged band was observed in other kinds of leukemia or in T-NHL patients. All of these patients with tald rearrangement had CD1-CD2+ CD4-CD7+ CD10-pheno-type. Of these, seven were classified as stage I thymic differentiation, and eight have survived for three to 59 months remission. Four of seven patients investigated had normal karyotypes, which has been reported to be associated with a good prognosis in T-cell ALL. We conclude that tald rearrangement is restricted to T-cell ALL, for which it provides a useful clonal marker. Such patients with this rearrangement may constitute a subgroup of T-cell ALL with a good prognosis.