We report a case of a patient who presented with clinical and radiological features of a non-resolving pneumonia. Special investigations and a poor response to antibiotic therapy excluded an infective aetiology. A diagnosis of invasive mucinous adenocarcinoma, previously termed bronchioloalveolar carcinoma, was made from lung biopsy. This case illustrates the challenges of establishing a timely diagnosis of an uncommon pneumonic-type of adenocarcinoma.