Although inhibition of epidermal growth factor receptor (EGFR)-mediated cell signaling by the EGFR tyrosine kinase inhibitor gefitinib is highly effective against advanced non-small cell lung cancer, this drug might promote severe acute interstitial pneumonia. We previously reported that molecular hydrogen (H 2) acts as a therapeutic and preventive antioxidant. Here, we show that treatment with H 2 effectively protects the lungs of mice from severe damage caused by oral administration of gefitinib after intraperitoneal injection of naphthalene, the toxicity of which is related to oxidative stress. Drinking H 2 −rich water ad libitum mitigated naphthalene/gefitinib-induced weight loss and significantly improved survival, which was associated with a decrease in lung inflammation and inflammatory cytokines in the bronchoalveolar lavage fluid. Naphthalene decreased glutathione in the lung, increased malondialdehyde in the plasma, and increased 4-hydroxy-2nonenal production in airway cells, all of which were mitigated by H 2-rich water, indicating that the H 2-rich water reverses cellular damage to the bronchial wall caused by oxidative stress. Finally, treatment with H 2 did not interfere with the antitumor effects of gefitinib on a lung cancer cell line in vitro or on tumor-bearing mice in vivo. These results indicate that H 2rich water has the potential to improve quality of life during gefitinib therapy by mitigating lung injury without impairing anti-tumor activity.