The expression of the natural killer (NK) cell marker CD56 has been reported to occur in NK cell lymphomas/leukemias and a small group of peripheral T-cell lymphomas but has not been studied extensively in primary intestinal non-B-cell lymphomas. Normal human jejunal intraepithelial lymphocytes (IELs) are mainly T-cell receptor (TCR)-alphabeta+CD3+CD8+CD5low and include an approximately 15% fraction of CD56+ cells that could be the cells of origin for CD56+ intestinal T-cell lymphoma (ITL). To test this hypothesis, 70 cases diagnosed as ITL were immunophenotyped, and 15 CD56+ cases (21%) were identified. The majority of the CD56+ lymphomas was of monomorphic small to medium-sized histology, shared the common phenotype betaF1+/-CD3epsilon/cyt+CD8+CD4-CD5-CD57-TIA-1+ and had clonally rearranged TCR gamma-chain genes. In contrast, the CD56- lymphomas were mainly composed of pleomorphic medium and large cells or had a morphology most consistent with anaplastic large-cell lymphoma and were mostly CD8-. These findings suggest that the majority of CD56+ intestinal lymphomas are morphologically and phenotypically distinct T-cell lymphomas most likely derived from activated cytotoxic CD56+CD8+ IELs. Some overlapping histological and clinical features between CD56+ and CD56- ITLs indicate that the former belong to the clinicopathological entity of ITL. The consistent expression of cytotoxic-granule-associated proteins introduces ITL (both CD56+ and CD56-) into the growing family of usually aggressive extranodal lymphomas of cytotoxic T-cell and NK-cell derivation. In contrast to putative NK-cell lymphoma of the sinonasal region, intestinal NK-cell lymphoma seems to be very rare.