This 52-year-old male ex-smoker presented with a six-month history of progressive breathlessness and weight loss. He deteriorated acutely, and was admitted with severe type 1 respiratory failure. Apart from diffuse coarse crackles on chest auscultation, physical examination was unremarkable. High-resolution computed tomography (HRCT) showed diffuse cystic changes throughout the lungs. A diagnosis of pulmonary Langerhans cell histiocytosis (PLCH) was considered. Further workup identified a coincidental pancreatic lesion of uncertain significance, which remained indeterminate on magnetic resonance imaging (MRI) and on positron emission tomography (PET). Transbronchial biopsy revealed enteric differentiated adenocarcinoma exhibiting lepidic spread, and autopsy later confirmed primary pancreatic malignancy. This case demonstrates that metastatic pancreatic malignancy can present with severe respiratory failure and masquerade as cystic lung disease.