The immunological and immunohistochemical characteristics of a patient with nodular lymphoid hyperplasia (NLH) of the terminal ileum were investigated pre- and post-operatively. The patient presented with diarrhea, lower abdominal pain, and weight loss, and an abdominal X-ray following barium enema revealed multiple small nodules in the terminal ileum which were subsequently confirmed to be NLH by histological examination. Preoperatively, although the serum immunoglobulin levels were normal, the T-cell responses in the peripheral blood, including the phytohemagglutinin (PHA)-blastogenesis and the T4/T8 ratio, were impaired. However, following an ileocecal resection, the PHA-blastogenesis became augmented and the T4/T8 ratio recovered to within the normal range. The immunohistochemical findings revealed a more marked accumulation of T cells in the interstitium around the hyperplastic follicles than in the interstitium of the normal intestine.