Plastic bronchitis is a rare lung disease characterized by the formation of tracheobronchial casts in the airways. These casts are thick and rubbery and consist of mucinous material (1-3). This condition is usually associated with systemic diseases (2). ln most cases, the disease can be diagnosed by chest X-ray and bronchoscopy. ln our case report, we present a 2-year-old boy with a negative past medical history. He was treated in primary care for dyspnea, cough and feveri but did not improve. His chest X-ray showed partial distelectasis in the lelt lung segments and compensatory hyperinflation in the right lung segments. The radiological findings were consistent with aspiration or pneumonia. The detailed history did not support aspiration ol a loreign body into the airway. Because ol the suspicion of pneumonia, we started a course of antibiotics and mucolytics. However, after one week of therapy, the condition ol the left lung progressed. We decided to perform bronchoscopy. We started the examination with fiberoscopy because of the possibility of chronic foreign body aspiration. Bronchoscopy was performed, we saw a tracheobronchial cast in the left upper lobe, which was blocked down the bronchial tree. We removed the cast by rigid bronchoscopy. We diagnosed plastic bronchitis. We made a thorough examination, which proved infection wilh Mycoplasma pneumoniae. ln some cases 0ur thinking can be wrong because of the discrepancy of the chest X-ray. This case calls our attention that in the same persistent radiological discrepancies we must perform the bronchoscopy.  Kulcsszavak: plastic bronchitis; chest X-ray; cast; bronchoscopy