A rare case of paraneoplastic cerebellar degeneration (PCD) in a 36-year-old woman is reported. She developed hyposthenia of the inferior limbs, diplopia, and disequilibrium in July 2001. Routine blood tests, tumoral markers, brain MRI, evoked potentials, and cerebrospinal fluid (CSF) examination were substantially normal. The clinical syndrome rapidly worsened in the following 2 months; she was wheelchair-bound with marked limb ataxia. CSF showed an increase of the IgG index with oligoclonal bands; brain MRI remained negative. The patient's serum and CSF were analyzed to detect antineuronal antibodies; anti-Yo antibodies were found that is typical of PCD. No tumor was found until April 2003; repeated CT scan, ultrasound, and mammographic examinations were negative. A further worsening in clinical symptoms was observed with a complete loss of autonomy (Rankin score 5) despite the performance of immunosuppressive therapy. In April 2003, an F-18 FDG PET scan visualized an area of abnormal uptake in the upper outer quadrant of the left breast. Interestingly, brain F-18 FDG uptake was normal. Suspicious microcalcifications were found on a new mammography and malignant cells were disclosed at cytology. The patient was operated on and final histologic examination revealed an infiltrating ductal breast cancer. In the reported case, F-18 FDG PET played a crucial role in detecting the unknown primary tumor in a young patient with PCD.