We report two cases of peritonitis carcinomatosa resulting from lung cancer. Case 1: A 70-year-old woman, diagnosed with advanced lung cancer, received platinum-based chemotherapy. It was ineffective, and she developed abdominal distension. She was admitted to our hospital where we diagnosed her with peritonitis carcinomatosa. Three weeks after receiving gefitinib, she was discharged and the treatment was continued for 9 months in an outpatient clinic. Case 2: A 55-year-old woman admitted to our hospital was suffering from peritonitis carcinomatosa and deep vein thrombosis. She was diagnosed with lung cancer 20 months prior to admission and had received chemotherapy. She underwent inferior vena cava filter placement after admission and commenced gefitinib. We frequently aspirated her ascites, even though she was receiving gefitinib. Unfortunately she died after 2 months. We encountered rare cases of peritonitis carcinomatosa resulting from lung cancer where the prognosis was very poor.