Differential diagnoses of various types of thoracic cystic masses are a prerequisite for appropriate management strategies. Mediastinal pancreatic pseudocysts are rare but life-threatening complications of pancreatitis. They may appear as a thoracic mass on X-rays and need to be differentiated from other lesions. This report demonstrates that magnetic resonance cholangiopancreaticography, which is not included in current algorithms, is a valuable diagnostic tool in such cases. We summarize the advantages and disadvantages of magnetic resonance cholangiopancreaticography and show how pre-interventional identification of pancreatic origin and exclusion of necrotic debris guided the therapeutic management of the patient. However, after 3 weeks of successful percutaneous drainage, infectious complications necessitated surgical intervention.