Loss of expression of the FHIT tumor suppressor gene is common in epithelial malignancies such as lung, kidney, esophageal, gastric, and cervical cancers. To assess the role of FHIT in acute leukemias, we examined 18 primary acute lymphoblastic leukemias (ALLs), 8 ALL-derived cell lines, 7 cell lines from other hematological malignancies, 14 lymphoblastoid cell lines, and 5 peripheral blood lymphocyte samples for expression of FHIT mRNA and protein by reverse transcription-PCR and Northern and Western blots. Fhit protein expression was detected in only 24% of primary ALLs and leukemia/lymphoma cell lines, but it was detected in all lymphoblastoid cell lines and peripheral blood lymphocyte samples. Interestingly, Fhit protein expression was lost in all T-cell ALLs but was lost in only half of the B-cell ALLs. Northern blotting of 7 normal lymphoblastoid cell lines and 13 of the neoplastic cell lines confirmed the results obtained by Western blotting regarding FHIT expression. The high frequency of loss of Fhit expression in ALLs suggests that inactivating alterations at the FHIT locus contribute to development of the leukemias.