Case 1 was a 69-year-old man with stage IV large cell carcinoma of the lung. A frontline chemotherapy regimen with carboplatin, paclitaxel, and bevacizumab was started. He was then offered a second treatment with pemetrexed (PEM). After 2 cycles, he presented exertional dyspnea. He was diagnosed as having pemetrexed-induced interstitial pneumonitis and was admitted to our hospital. During hospitalization, he had swelling and stiffness of bilateral multiple joints of extremities. Laboratory data from a blood sample showed elevated white blood cells (WBCs), C-reactive protein (CRP), and rheumatoid factor (RF), as well as positive titer for anti-cyclic citrullinated peptide (anti-CCP) antibody. He was diagnosed as having rheumatoid arthritis (RA), and treatment with prednisolone was started. Consequently, the interstitial pneumonitis on chest X-ray and polyarthritis improved. Case 2 was a 77-year-old woman with stage IV adenocarcinoma of the lung. The disease continued progress nine months after starting therapy with gefitinib, and she was offered a second treatment with carbo­platin and PEM. After 4 cycles of the chemotherapy regimen, she presented polyarthritis. Laboratory data showed elevated WBCs, CRP, and RF, as well as positive titer for anti-CCP antibody. She was diagnosed as having RA. Treatment with prednisolone and salazosulfapyridine was started, and the polyarthritis improved. These 2 cases suggest that anticancer chemotherapy with PEM may induce RA.