A 50-year-old female patient, who had been followed for 15 years for protein-losing enteropathy, was hospitalized due to epigastric pain. Examination on admission revealed that the patient was in the sub-ileus state. On the 26th day after admission, she complained of severe abdominal pain and shortly after she went into shock. The emergency laparotomy documented intestinal perforation and a tumor. The perforated site was right at the middle of tumor. The histological and histochemical studies identified the tumor as malignant lymphoma of B lymphocyte lineage. As far as we know, this is the third case of malignant lymphoma occurring in the jejunum in a patient with protein-losing enteropathy in Japan. The possible relationship between lymphomas and protein-losing enteropathy is discussed.