We had a 2 month, 5.8 kg male infant for a surgery of perianal fistula. He had no considerable past or family history or laboratory data except mild jaundice and abnormal liver function tests. He was intubated after intravenous injection of ketamine and vecuronium. Anesthesia was maintained with isoflurane-N2O and O2. The operation was over within just 10 minutes without any specific event. We found abdominal distension, then suctioned out about 100 ml of milkish, bloody, gastric juice. In recovery room, the patient was cyanotic and a hazziness of the whole lung field was found in the chest X-ray. Acute respiratory failure (ARF) developed and we suspected aspiration. The next day he was diagnosed with congenital hypertrophic pyloric stenosis (CHPS) in sonographic examination. His ARF improved and he recieved pyloromyotomy 2 weeks later. CHPS is a common gasrointestinal disorder requiring operation in the infant stage. We report the ARF of an infant with asymptomatic unrecognized CHPS for anal fistulectomy. The incidence and sonographic features of CHPS and problems associated with anesthetic concerns are discussed.