Human X-linked severe combined immunodeficiency disease (SCID) is an immunodeficiency disorder in which T cell development is arrested in the thymic cortex. B lymphocytes in children with X-linked SCID seem to differentiate normally. X-linked SCID is associated with a mutation in the gene that encodes the IL-2R γ-chain. Because TCR-β gene recombination is a pivotal initial event in T lymphocyte ontogeny within the thymus, we hypothesized that a failure to express normal IL-2Rγ could lead to impaired TCR-β gene recombination in early thymic development. PCR was used to determine the status of TCR-β gene-segment rearrangements in thymic DNA that had been obtained from children with X-linked SCID. The initial step in TCR- β gene rearrangement, that of Dβ to Jβ recombination, was readily detected in all thymus samples from children with X-linked SCID; in contrast, Vβ to DJβ gene rearrangements were undetectable in the same samples. Both Dβ to Jβ and Vβ to DJβ TCR genes were rearranged in the thymic tissues obtained from immunologically normal children. We conclude that TCR β-chain gene rearrangement is arrested in children with X-linked SCID. Our results suggest a causative relationship between the failure of TCR β-chain gene rearrangements to proceed beyond DJβ rearrangements and the production of a nonfunctional IL-2R γ-chain.