Caspase 10 (Mch4/FLICE2) is a caspase homologous to caspase 8. A recent report described that inherited CASP10 gene mutations underlie defective lymphocyte and dendritic cell apoptosis in autoimmune lymphoproliferative syndrome (ALPS). In this study, to explore the possibility that mutation of this gene might be involved in the development of non-Hodgkin lymphoma (NHL), we have analyzed the entire coding region and all splice sites of the CASP10 gene for the detection of somatic mutations in 117 human NHLs. Overall, 17 NHLs (14.5%) were found to have CASP10 mutations, which were identified in the coding regions of the prodomain (n = 3), the p17 large protease subunit (n = 11), and the p12 small protease subunit (n = 3). We expressed the tumor-derived caspase 10 mutants in 293 cells and found that apoptosis was suppressed. These data suggest that the inactivating mutations of the CASP10 gene might lead to the loss of its apoptotic function and contribute to the pathogenesis of some human NHLs.