Chemotherapy-induced oral mucositis (CIOM) is a severe adverse event resulting from cancer chemotherapy. Toxic free radicals and pro-inflammatory cytokines produced by anticancer drugs have been reported to be associated with CIOM. Rebamipide has been shown to increase gastric endogenous prostaglandin E2 and I2, to promote gastric epithelial mucin, and to behave as an oxygen free-radical scavenger in addition to other anti-inflammatory actions. We developed a gargle solution of rebamipide, adding ultrahydrogel for mucosal protection and to maintain rebamipide on the oral mucosa. A 300 mL rebamipide gargle solution combines 600 mg rebamipide, 3 g high molecular-weight polyethylene oxide, 1.2 g carrageenan, pineapple flavoring, and water. The efficacy of the rebamipide gargle was evaluated in 175 patients with CIOM from November 2009 to December 2012, each instructed to use the rebamipide gargle 5-6 times daily. The severity of CIOM was assessed according to the National Cancer Institute's Common Terminology Criteria for Adverse Events (CTCAE, version 4.0). Their CTCAE scores (3/2/1/0) changed from n=13/64/98/0 to 0/10/103/62, respectively, after initiation of the rebamipide gargle (p<0.01; paired t-test). The median duration to best response was 14 days (range: 1-49). CTCAE scores decreased in 132 patients (75.4%), including 62 (35.4%) who achieved grade 0. There were no unexpected safety events. Rebamipide gargle was well tolerated and demonstrated to have significant therapeutic efficacy against CIOM.