A case of familial adenomatosis coli with large intra-abdominal desmoid tumors is described. The patient initially complained of an abdominal mass, which was diagnosed as a desmoid tumor after surgical resection. Further examinations revealed numerous polyps of the colon which were histologically diagnosed as adenomas. Despite intensive medical therapy, the desmoid tumors rapidly increased in size and number, and resulted in abscess and cavity formation that involved the ileum. Although desmoid tumors are classically benign in nature, we consider these tumors to be a therapeutic problem in patients with familial adenomatosis coli.