The results of treatment of 40 cases of primary malignant tumor of the small intestine are reported. The malignant tumors included 14 leiomyosarcoma, 12 malignant lymphoma and 14 adenocarcinoma. Our standard treatment for these tumors was segmental resection of the small intestine with its mesentery including the regional lymph nodes and we used additional adjuvant chemotherapy for some cases of malignant lymphoma and adenocarcinoma. Curative resection was performed in 57.1% of leiomyosarcoma, 50.0% of malignant lymphoma and 61.5% of adenocarcinoma. Cumulative 5-year survival rates were leiomyosarcoma 21.4%, malignant lymphoma 53.0% and adenocarcinoma 43.3% respectively. Serosal infiltration, lymph node metastasis and operative curability proved to be the prognostic factors. In particular, the 5-year survival rate of patients on whom curative resection could be performed was significantly better for each tumor type than that of patients for whom this was not possible, the figures being leiomyosarcoma 40.0%, malignant lymphoma 83.3% and adenocarcinoma 83.3%.