Introduction: Differential diagnostics of cystic cervical neoplasia is sometimes difficult. Genuine branchiogenic carcinoma, which means carcinoma in lateral cysts of the neck, are rare. Case presentation: A 51 year old patient presented with a 4-week history of a progressive swelling postero-inferior to the right mandibular angle. Ultrasound of the neck showed a well defined, shmoothly shaped and homogenous cystic mass without hilus sign. A computed tomography scan of the head and neck revealed a 40mm hypodense mass in Level IIa, most likely a central necrotic lymph node metastasis. An 18FDG PET-CT scan revealed a rounded, metabolic active mass with central hypodensity in the right carotid triangle. There was no clinical or radiographic sign for any primary cancer. The mass was excised. Result: Microscopic examination revealed a branchiogenic carcinoma. We applied a functional neck dissection on the right side and a bilateral tonsillectomy. The right tonsil contained a 2,2cm squamous cell carcinoma which infiltrated almost the whole tonsil (pT2, pN2a, M0). Discussion and Conclusion: On assumption of a Cancer of unknown primary (CUP) we performed a PET-CT scan, which is clearly superior to single PET, CT or MRI. Despite the size of 2,2cm the primary cancer could not clearly be determined. First histologic examination revealed a branchiogenic carcinoma, which would be a rarity. Proof was given through diagnostic tonsillectomy, which is recommended for CUP of the head and neck but not in branchiogenic carcinoma. It has to be clarified whether branchiogenic carcinoma is a unique entity. Keywords: branchiogenic, carcinoma, metastasis, neck, tonsil