Pneumatosis cystoides intestinalis (PCI) is a relatively rare condition characterized by intraluminal gas in the gastrointestinal tract. Several chemotherapeutic agents have been reported to be associated with PCI, although fluorouracil-related PCI is extremely rare. We report a case of a 76-year old man who received adjuvant chemotherapy for rectal cancer with fluorouracil (FU) and leucovorin (LV). After 1 cycle of the treatment, he presented with diarrhea and abdominal pain. Abdominal radiogram revealed the presence of free air under the diaphragm and intramural gas in the intestine. Laparotomy was performed, showing a suspected diagnosis of perforation in the gastrointestinal tract. Intraoperative findings revealed penumatosis of the intestine without evidence of perforation. He was treated supportively and his symptoms improved. In conclusion, we should consider the possibility of PCI occurring in patients with malignancies during chemotherapy treatment. Several chemotherapeutic agents have been reported to be associated with PCI, although fluorouracil-related PCI is extremely rare. We report a case of a 76-year old man who received adjuvant chemotherapy for rectal cancer with fluorouracil (FU) and leucovorin (LV). After 1 cycle of the treatment, he presented with diarrhea and abdominal pain. Abdominal radiogram revealed the presence of free air under the diaphragm and intramural gas in the intestine. Laparotomy was performed, showing a suspected diagnosis of perforation in the gastrointestinal tract. Intraoperative findings revealed penumatosis of the intestine without evidence of perforation. He was treated supportively and his symptoms improved. in patients with malignancies during chemotherapy treatment.