We analyzed the usage of the delta recombining element (delta Rec) and six V delta genes in cell samples from 15 patients with CD3- and 10 patients with CD3+ T-cell acute lymphoblastic leukemia in an attempt to define the hierarchy of genetic events that is associated with the T-cell receptor (TCR) alpha/delta gene complex during T-cell ontogeny. Based on the deletion patterns of these genes, we surmised their relative order on chromosome 14 to be as follows: 5'-V delta 4, V delta 6, V delta 1, V delta 5, delta Rec, V delta 2, D delta 1-3, J delta 1-3, C delta, V delta 3-3'. In agreement with previous reports, V delta 1 was found to be preferentially rearranged in CD3+ samples. In CD3- samples, V delta 2 and V delta 3 rearrangements were observed at a high frequency. Incomplete V delta D delta rearrangements using V delta 2 or V delta 3, which are closest to C delta, were observed in three patients with CD3- and one patient with CD3+. These results suggest that V delta 2- and V delta 3-(Dn)D delta 3 recombinations are among the earliest recombinational events. Delta Rec was observed to be rearranged to phi J alpha on one allele. In addition, delta Rec rearrangements to J delta 1 and J alpha close to phi J alpha were also demonstrated on three alleles and one allele, respectively. Delta Rec rearrangements to J delta and J alpha other than phi J alpha also inhibit expression of the TCR delta locus. Approximately half of the alleles with J delta rearrangements showed no involvement of known V delta or delta Rec, indicating the existence of other, yet-uncharacterized V delta or delta Rec-like segments.