A 57-year-old man had had schizophrenia since 1979. He had a history of noncompliance with medications. In November 2010, he was admitted to our hospital and diagnosed as pneumonia. Piperacillin/tazobactam and ciprofloxacin were administered, but clinical symptom and laboratory data were not improved. We suspected drug-induced pneumonitis because chest CT scans revealed diffuse bilateral ground-glass opacities and the elevation of serum KL-6. After discontinuation of levomepromazine and chlorpromazine and the administration of prednisolone (50 mg daily), his physical examination and clinical data were improved. But after chlorpromazine was readministered accidentally by a psychiatrist, he had a temperature of 38.4°C, and his chest radiograph was deteriorated. He was diagnosed as chlorpromazine-induced pneumonitis. After discontinuancee of the chlorpromazine, clinical data and chest radiograph were improved. Prednisolone was tapered to 15 mg gradually, and he was discharged from our hospital 41 days after admission. To the best of our knowledge, there are few cases of phenothiazine-induced pneumonitis.