A female patient with metastatic mucoepidermoid carcinoma found in pulmonary lymphatics received 18F-FDGpositron emission tomography(PET)for detection of unknown primary tumor after unsuccessful conventional diagnostic workup. In addition to multiple hypermetabolic lesions in the right neck, right mediastium and right lung, intense focal uptake was identified at left upper quadrant of the abdomen, which was suspected to be the primary site of malignancy. However, the pathology revealed benign tubovillous adenoma of the colon. The primary tumor has yet to be detected. In this case, FDGPET failed to identify the primary tumor.