Enforced BCL-2 gene expression in leukemic cell lines suppresses apoptosis and confers resistance to anticancer drugs, but the clinical significance of increased BCL-2 protein levels in acute lymphoblastic leukemia (ALL) is unknown. Among 52 children with newly diagnosed ALL, BCL-2 expression in leukemic lymphoblasts ranged widely, from 4,464 to 59,753 molecules of equivalent soluble fluorochrome per cell (MESF), as determined by flow cytometry. The mean (+/- SD) level of MESF in 43 cases of B-lineage ALL (19,410 +/- 11,834) was higher than that detected in CD10+ B-lymphoid progenitors from normal bone marrow (450 +/- 314; P < .001), and CD19+ peripheral blood B lymphocytes (7,617 +/- 1,731; P = .02). Levels of BCL-2 in T-ALL cases (17,909 +/- 18,691) were also generally higher than those found in normal CD1a+ thymocytes (1,762 +/- 670), or in peripheral blood T lymphocytes (9,687 +/- 3,019). Although higher levels of BCL-2 corresponded to higher leukemic cell recoveries after culture in serum-free medium, they did not correlate with higher cell recoveries after culture on stromal layers, or with in vitro resistance to vincristine, dexamethasone, 6- thioguanine, cytarabine, teniposide, daunorubicin or methotrexate. BCL- 2 protein levels did not correlate with presenting clinical features. Unexpectedly, however, lower-than-median MESF values were significantly associated with the presence of chromosomal translocations (P = .010). Notably, all six cases with the Philadelphia chromosome, a known high- risk feature, had low levels of BCL-2 expression (P = .022). Higher levels of BCL-2 were not associated with poorer responses to therapy among 33 uniformly treated patients, and were not observed in three patients studied at relapse. In conclusion, increased BCL-2 expression in childhood ALL appears to enhance the ability of lymphoblasts to survive without essential trophic factors, and is inversely related to the presence of chromosomal translocations. However, it does not reflect increased disease aggressiveness or resistance to chemotherapy.