F-18 fluorodeoxyglucose positron emission tomography (FDG PET) is highly sensitive and specific for the detection of nodal and extranodal disease in patients with malignant lymphoma. However, no previous reports have shown lymphomatous involvement of the small bowel that was recognized on the basis of FDG-PET imaging findings. The authors present two cases of clinically unsuspected small bowel involvement by mantle cell lymphoma that were initially detected on FDG PET.