OBJECTIVE: To estimate the associations of acidity and concentration of selected minerals in household tap water with the risk of type 1 diabetes. RESEARCH DESIGN AND METHODS: We designed a population-based case-control study with 64 cases of type 1 diabetes and 250 randomly selected control subjects. Acidity, color, and mineral content were measured in tap water from each participant's household. RESULTS: Tap water pH 6.2-6.9 was associated with a fourfold higher risk of type 1 diabetes compared with pH > or =7.7 (OR 3.73, 95% CI 1.52-9.15). This result was similar after exclusion of individuals with the highly protective HLA-DQB1*0602 allele, but adjustment for maternal education, urban/rural residence, sex, and age tended to strengthen the estimated association. Higher tap water concentration of zinc was associated with lower risk of type 1 diabetes after adjustment for pH and other possible confounders, but the overall association was strictly not significant. CONCLUSIONS: These results suggest the possibility that quality of drinking water influences the risk of type 1 diabetes. The possible mechanisms by which water acidity or mineral content may be involved in the etiology of type 1 diabetes remain unknown, but the mechanisms are most likely indirect and may involve an influence on survival of microorganisms in the water.