The prevalence of symptoms of atopic eczema among Japanese children aged 6-7 years is the second highest in 56 countries. Reasons for such a high prevalence are unknown. This ecological study examined whether the positive association of water hardness with atopic dermatitis among British primary-school children also exists in Japan. Study subjects were 458,284 of 489,725 children, aged 6-12 years, in 1,016 public elementary schools in Osaka Prefecture with 44 municipalities. The study used data on water hardness and chlorine content of the water supply; prevalence of atopic dermatitis diagnosed by physicians and episodes of wheezing reported by the parents; and potential confounding factors by socioeconomic and health care status per municipality. The prevalence of atopic eczema was significantly higher in the highest water hardness category than that in the lowest (24.4% [n=2,288/9,362] and 22.9% [n=77,408/33,8267], respectively; adjusted odds ratio 1.12, 95% confidence interval 1.06-1.18; P<0.0001 for trend). A significant and slightly J-shaped relationship between chlorine content of the water supply and prevalence of atopic dermatitis was observed after adjustment for confounding factors. There were no apparent associations of water hardness and chlorine content of the water supply with prevalence of wheeze. Water hardness may increase the risk of atopic dermatitis among elementary-school children in Japan, as well as in the United Kingdom.