We analyzed the rearrangement and expression of T-cell receptor (TCR) genes, including the recently identified TCR delta gene, in 21 patients with T-lineage leukemia/lymphoma. Among 8 patients with CD3-, CD4-, and CD8- (group I), 2 patients showed germline configuration of the TCR delta, gamma, beta, and alpha genes and 1 patient demonstrated only TCR delta gene rearrangement. All nine patients with CD3-, CD4+, and/or CD8+ (group II) showed concomitant rearrangements of the TCR delta, gamma, and beta genes. TCR alpha gene rearrangement was also observed in two patients. Three of four patients with CD3+ (group III) showed rearrangement of the TCR alpha gene with deletion of both alleles or of a single allele of the TCR delta gene. With Northern blot analysis, full-length transcripts of the TCR delta gene were detected in 3 of 15 examined patients. All were restricted to group I or group II. In contrast, full-length transcripts of TCR beta and alpha were observed mainly in samples from groups II and III. Based on these findings, rearrangement of the TCR delta gene may be the earliest event in T-cell differentiation, preceding rearrangements of the other TCR genes.