A 32-year-old male with CATCH22 syndrome presented with a high fever and productive cough after taking drugs for acute bronchitis, including (L)-carbocisteine. Chest radiography revealed ground-glass opacities in the bilateral lung fields. He had a history of similar pneumonia. Under the assumption of drug-induced pneumonia, or bacterial or viral pneumonia, all drugs including (L)-carbocisteine were discontinued, and antibiotics were started. A drug-induced lymphocyte stimulation test was positive only for (L)-carbocisteine. The only drug in common between this and the previous episode of pneumonia was (L)-carbocisteine. We thus concluded that this was a definite case of (L)-carbocisteine-induced pneumonia in a patient with CATCH22 syndrome.