In patients with anti-Yo associated paraneoplastic cerebellar degeneration (PCD) neurological symptoms precede the diagnosis of the underlying cancer in about 60%. Ovarian carcinoma, breast cancer and other gynaecological malignancies are most frequently found as causative malignancies. Antitumour treatment should be applied in an early stage of disease. The identification of the tumour is a diagnostic challenge in many of these patients. In the first of two patients reported here a pelvic tumour was suggested after detection of a pathological lymph node and elevated tumour markers. The intraoperative findings appeared macroscopically normal during ovariectomy with adnexectomy. Not until microscopic examination of the resected tissue was performed was a tubal adenocarcinoma found. If intrapelvic gynaecological tumours are suspected a deliberate surgical exploration seems to be justified, but only after an intensive diagnostic investigation. To search for the underlying cancer in patients with paraneoplastic neurological disorders successive CT and [18F]-FDG-PET are widely recommended. Instead of this in the second reported patient whole-body dual-modality PET/CT was performed revealing enhanced uptake in three regions of the left thorax. By combining function and anatomy PET/CT was able to localise the lesions and characterise them as lymph node metastases of breast cancer. Diagnosis could be confirmed by subsequently executed needle biopsy. PET/CT seems to be highly applicable in the investigation of paraneoplastic disorders with unknown primary cancer. It may help in guidance of needle biopsy or to optimise the results of deliberate surgery and it provides whole-body tumour staging in a single session with higher diagnostic accuracy than PET alone.