The identification of genetic risk subgroups of T-cell acute lymphoblastic leukemia (T-ALL) may provide evidence for risk stratification and individualized treatment. We investigated the characteristics and prognostic value of tumor suppressor gene CDKN2A deletions in 101 patients with T-ALL. The CDKN2A deletion was present in 23% (23/101) of T-ALL by fluorescence in situ hybridization (FISH). The most common type of CDKN2A deletion was homozygous deletion (70%, 16/23). A lower frequency of CDKN2A deletion was found in patients with early T-cell precursor (ETP) ALL than in patients with non-ETP-ALL (10.4% vs 34.0%; P = .008). Deletion of CDKN2A was significantly associated with younger age (P = .001), higher white blood cell (WBC) count (P < .001) and higher lactate dehydrogenase (LDH) level (P = .002). Patients with CDKN2A deletion had lower 2-year overall survival (OS) and event-free survival (EFS) rates than patients without CDKN2A deletion (2-year OS: 18.6% ± 8.9% vs 47.4% ± 6.2%, P = .032; EFS: 16.4 ± 8.3 vs 38.6 ± 5.9%, P = .022). In multivariable analysis, CDKN2A deletion was an independent adverse prognostic factor for OS (P = .016). In conclusion, adult T-ALL patients with CDKN2A deletion had a poor prognosis, and these patients might benefit from intensive chemotherapy or allogeneic hematopoietic stem-cell transplantation.