The diagnosis of cancer is often difficult in patients with paraneoplastic neurological syndrome and anti-Hu antibodies. Fluorodeoxyglucose 18 positron emission tomography scanning is a highly sensitive and specific method to detect lung tumors. We investigated 15 patients with paraneoplastic neurological syndrome and anti-Hu antibodies. Radiological methods led to the diagnosis of cancer in 12 patients, and test results were negative in 3. Whole-body [18F]fluorodeoxyglucose positron emission tomography showed abnormal uptake in the mediastinum in these 3 patients in accordance with the expected location of the malignancy.