TEL/AML1 (also known as ETV6/RUNX1) rearrangement is the most frequent genetic change in childhood B-acute lymphoblastic leukemia (ALL) and is associated with a favorable prognosis. Its presence in adult ALL, however, has been questionable, and any association between TEL/AML1 rearrangement and clinical prognosis is unknown. To reveal the presence and incidence of the TEL/AML1 rearrangement in adult ALL, we applied fluorescence in situ hybridization (FISH). We conducted extra-signal, dual-color fluorescence in situ hybridization (ES-FISH) for TEL/AML1 rearrangement on bone marrow cells from 74 adult ALL patients and analyzed the survival time. We demonstrated the TEL/AML1 rearrangement in 2 patients (2.7%) among 74 patients with ALL. Of 74 adult ALL patients, 3 (4.0%) showed deletion of the TEL gene without TEL/AML1 rearrangement. The mean survival time of patients with TEL/AML1+ALL and patients with cryptic TEL deletion was 33 and 5 months, respectively, whereas the mean survival time of 53 TEL/AML1-ALL patients was 16.7 months. TEL/AML1 rearrangement is not unique in childhood ALL, and cryptic TEL deletion without TEL/AML1 rearrangement was more frequent than the TEL/AML1 rearrangement in adult ALL. We recommend TEL/AML1 FISH in adult ALL patients because conventional cytogenetic studies so far have yielded uninformative results.