Gangliocytic paraganglioma (GP) is a rare benign neuroendocrine tumour found most often in the duodenum. To our knowledge, only a dozen cases of possibly malignant duodenal GP with local lymph node metastasis and only one case with liver metastasis have previously been published. Herein, we report an unusual case of GP of the duodenum spreading to the retropancreatic space and metastatic to the liver and lymph nodes. Additionally, the present tumour secreted pancreatic polypeptide (PP) which was detected in the serum during the follow-up period. We suggest that serum PP could be a valuable marker in the diagnosis and follow-up of patients with GP.