In patients with anti-Yo associated paraneoplastic cerebellar degeneration (P CD) neurological symptoms precede the diagnosis of the underlying cancer in abou t 60%. Ovarian carcinoma, breast cancer and other gynaecological malignancies a re most frequently found as causative malignancies. Antitumour treatment should be applied in an early stage of disease. The identification of the tumour is a d iagnostic challenge in many of these patients. In the first of two patients repo rted here a pelvic tumour was suggested after detection of a pathological lymph node and elevated tumour markers. The intraoperative findings appeared macroscop ically normal during ovariectomy with adnexectomy. Not until microscopic examina tion of the resected tissue was performed was a tubal adenocarcinoma found. If i ntrapelvic gynaecological tumours are suspected a deliberate surgical exploratio n seems to be justified, but only after an intensive diagnostic investigation. T o search for the underlying cancer in patients with paraneoplastic neurological disorders successive CT and[18F]-FDG-PET are widely recommended. Instead of th is in the second reported patient whole-body dualmodality PET/CT was performed revealing enhanced uptake in three regions of the left thorax. By combining func tion and anatomy PET/CT was able to localise the lesions and characterise them a s lymph node metastases of breast cancer. Diagnosis could be confirmed by subseq uently executed needle biopsy. PET/CT seems to be highly applicable in the inves tigation 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 diagnosti c accuracy than PET alone. In patients with anti-Yo associated paraneoplastic cerebellar degeneration (P CD) neurological symptoms precede the diagnosis of the underlying cancer in abou t 60%. Ovarian carcinoma, breast cancer and other gynecological malignancies a re most frequently found as causative malignancies. Antitumor treatment should be applied in an early stage of disease. The identification of the tumor is ad iagnostic challenge in many of these patients. In the first of two patients repo rted here a pelvic tumor was suggested after detection of a pathological lymph node and elevated tumor markers. The intraoperative findings were the result of a resected tissue was performed a tubal adenocarcinoma. If i ntrapelvic gynaecological tumors are suspected a deliberate surgical exploratio n seems to be justified, but only after an intensive diagnostic investigation. T o search for the underlying cancer in patients with paraneoplastic neurological disorders successive CT and [18F] -FDG-PET are widely recommended. Instead of th is in the second reported patient whole-body dualmodality PET / CT was performed revealing uptake in three regions of the left thorax. By combining func tion 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 subseq uently executed needle biopsy. PET / CT seems to be highly applicable in the inves tigation of paraneoplastic disorders with unknown primary cancer. It may help in guidance of needle biopsy or to optimize the results of deliberate surgery and it provide whole-body tumor staging in a single session with higher diagnosti c accuracy than PET alone.