A 69-year-old man with massive ascites was referred to our hospital. Paracentesis revealed exudative ascites with many abnormal lymphocytes, which expressed T-natural killer (T-NK) cell surface markers and gamma deltaT-cell receptor (TCR). Although the ascites resolved for a short time with chemotherapy, gastrointestinal bleeding occurred and acute retention of ascites was observed. The patient died of panperitonitis. At autopsy, part of the jejunum revealed ulceration and marked mucosal thickening, and was perforated at the site of the severe ulceration. Histological examination showed massive infiltration of abnormal lymphocytes that were positive for CD45RO. Therefore, the cells responsible for the jejunal lymphoma and ascites were thought to be derived from gamma deltaIEL.