Here we report a 74-year-old man with squamous cell lung cancer who was administered pembrolizumab. The patient was admitted to our hospital because of frequent diarrhea and abdominal pain after 31 cycles of pembrolizumab. Given the clinical, radiological, endoscopic, and pathological findings, the patient was diagnosed with irAE colitis. The patient's symptoms improved with prednisolone after cessation of pembrolizumab. However, he developed aspiration pneumonia. After cessation of prednisolone and treating with antibiotics, he had pseudomembranous colitis and CMV colitis. Both metronidazole and ganciclovir were initiated. Then irAE colitis had recurred and oral 5-ASA was administered. Thereafter, the patient's general condition has improved and colonoscopy showed a slight rough mucosa but improved visible vascular pattern 6 months after the treatment of 5-ASA.
