Summary 　Since a prominent effect of molecular hydrogen on cerebral infarction has been reported in June 2007, effects of molecular hydrogen have been documented in 54 disease models in less than four years. Three human studies show effects of hydrogen on diabetes mellitus type 2, metabolic syndrome, and hemodialysis. However, three enigmas remain unsolved. First, a small amount of hydrogen has marked effects on rodents but not on cultured cells. Second, drinking hydrogen water is usually more effective than inhaling hydrogen gas, but should be able to bring a small amount of hydrogen to the human/rodent bodies. Third, humans and rodents produce a large amount of hydrogen gas in the large intestine, but an addition of a small amount of hydrogen exhibits prominent effects. Further studies are required to elucidate the molecular bases of hydrogen effects and to prove therapeutic and preventive effects of hydrogen on various human diseases.