Familial adenomatous polyposis (FAP) is an autosomal dominant disorder characterized by colonic polyposis and a predisposition for developing colorectal cancer. FAP is frequently complicated by extracolonic disease, but complications of leukemia are rare. We present the first case of FAP complicated by chronic myelogenous leukemia (CML) in a 38-year-old man. The patient had numerous adenomas in the colorectum and a family history compatible with FAP. He was diagnosed as having FAP in February 2000. Two years after the diagnosis, he developed leukocytosis with the Philadelphia chromosome abnormality, indicating complication with CML. Imatinib mesylate was administered for the treatment of CML, and hematologic and cytogenetic remission of CML was achieved in 6 months. Numerous polyps, 2 to 3 mm in diameter, observed in the rectum prior to the administration of imatinib, regressed in size, but not in number, after 1 year of treatment with imatinib. Eighteen months later, however, the polyps were enlarged. In this patient, imatinib administration led to the remission of CML and might also have been responsible for the temporary regression of adenomatous polyps of FAP.