Pneumatosis cystoides intestinalis (PCI) is a relatively rare condition, characterized by subserosal or submucosal air within the bowel wall. Herein, we report a rare case of PCI secondary to treatment with an epidermal growth factor receptor-tyrosine kinase inhibitor (EGFR-TKI). A 71-year-old man, who had received gefitinib therapy for 2 years and 5 months for lung adenocarcinoma with metastases to the bones and brain, presented with abdominal pain, diarrhea, and vomiting. Computed tomography of the abdomen revealed intramural air in the small bowel, free air in the abdomen, and moderate ascites. A diagnosis of PCI was made, and the patient was managed conservatively by discontinuing gefitinib treatment, because his vital signs were stable and there was no sign of peritonitis. The patient's symptoms gradually improved, and follow-up CT after 1 week revealed that the initial findings had almost completely resolved. Clinicians should note that treatment with gefitinib might cause PCI.