Oxidative stress is implicated in the pathogenesis of many diseases; however, currently used antioxidants have a high toxicity that constrains administration to a narrow window of therapeutic dosage. There is a clear need for more effective and safer antioxidants. Diatomic hydrogen (H(2)) was proposed as a novel antioxidant that selectively reduces levels of toxic reactive-oxygen species. Recently, many studies have reported that H(2) (inhaled or orally ingested, typically as approximately 0.8 mM H(2)-saturated water), can exert beneficial effects in diverse animal models of ischaemia-reperfusion injury, and inflammatory and neurological disease. In the clinic, oral administration of H(2)-saturated water is reported to improve lipid and glucose metabolism in subjects with diabetes or impaired glucose tolerance; promising results have also been obtained in reducing inflammation in haemodialysis patients and treating metabolic syndrome. These studies suggest H(2) has selective antioxidant properties, and can exert antiapoptotic, antiinflammatory and antiallergy effects. This review summarizes recent research findings and mechanisms concerning the therapeutic potential of H(2).