The Notch signaling pathway plays an important role in cellular proliferation, differentiation, and apoptosis. Unregulated activation of Notch signaling can result in excessive cellular proliferation and cancer. Human T-cell leukemia virus type 1 (HTLV-I) is the etiological agent of adult T-cell leukemia (ATL). The disease has a dismal prognosis and is invariably fatal. In this study, we report a high frequency of constitutively activated Notch in ATL patients. We found activating mutations in Notch in more than 30% of ATL patients. These activating mutations are phenotypically different from those previously reported in T-ALL leukemias and may represent polymorphisms for activated Notch in human cancers. Compared with the exclusive activating frameshift mutations in the proline, glutamic acid, serine, and threonine (PEST) domain in T-ALLs, those in ATLs have, in addition, single-substitution mutations in this domain leading to reduced CDC4/Fbw7-mediated degradation and stabilization of the intracellular cleaved form of Notch1 (ICN1). Finally, we demonstrated that inhibition of Notch signaling by γ-secretase inhibitors reduced tumor cell proliferation and tumor formation in ATL-engrafted mice. These data suggest that activated Notch may be important to ATL pathogenesis and reveal Notch1 as a target for therapeutic intervention in ATL patients.