A delayed diagnosis of tracheobronchial foreign body aspiration (FBA) may result in severe respiratory complications such as pneumonia, bronchiectasis, and atelectasis. Here, we present a rare case of a left lung destruction caused by a pen cap that remained in the left lower lobe bronchus for 25 years and was previously misdiagnosed as cavitary tuberculosis. The foreign body was not detected in bronchoscopic examination prior to surgery due to severe stenosis of the left main bronchus. The foreign body was found incidentally while the patient was undergoing left pneumonectomy. To the best of our knowledge, this the second longest delay to diagnosis of FBA reported. This case also exemplifies the grave consequences of misdiagnosis of FBA.