Mutations in ATM (ataxia-telangiectasia mutated) cause ataxia-telangiectasia (AT), a disease characterized by neurodegeneration, sterility, immunodeficiency, and T-cell leukemia. Defective ATM-mediated DNA damage responses underlie many aspects of the AT syndrome, but the basis for the immune deficiency has not been defined. ATM associates with DNA double-strand breaks (DSBs), and some evidence suggests that ATM may regulate V(D)J recombination. However, it remains unclear how ATM loss compromises lymphocyte development in vivo. Here, we show that T-cell receptor β (TCRβ)–dependent proliferation and production of TCRβlow CD4+CD8+ (DP) thymocytes occurred normally in Atm−/− mice. In striking contrast, the postmitotic maturation of TCRβlow DP precursors into TCRβint DP cells and TCRβhi mature thymocytes was profoundly impaired. Furthermore, Atm−/− thymocytes expressed abnormally low amounts of TCRα mRNA and protein. These defects were not attributable to the induction of a BCL-2–sensitive apoptotic pathway. Rather, they were associated with frequent biallelic loss of distal Va gene segments in DP thymocytes, revealing that ATM maintains Tcra locus integrity as it undergoes V(D)J recombination. Collectively, our data demonstrate that ATM loss increases the frequency of aberrant Tcra deletion events, which compromise DP thymocyte maturation and likely promote the generation of oncogenic TCR translocations.