Malignant tumors of the small bowel are rare and little is known about their etiology, although adenocarcinomas share certain epidemiological features with colorectal cancer. This study investigated what cancers, if any, occurred as second neoplasms following adenocarcinomas, malignant carcinoid tumors, lymphomas, and sarcomas of the small bowel. For all 2581 cases of small bowel malignancy registered in one of the Surveillance, Epidemiology, and End-Results program areas, 1973-1988, the relative risk of a second malignancy was determined. The risk of colorectal cancer was increased following adenocarcinoma of the small bowel, and the risk of adenocarcinoma of the small bowel was increased following colorectal cancer in both males and females. This study also found an association between small bowel sarcomas and malignant melanoma in males, consistent with earlier studies, and an association between prostate cancer and malignant carcinoid tumors of the small bowel, a new observation. We conclude that adenocarcinomas of the small bowel may share risk factors with colorectal cancer.