In recent years, several studies have addressed a possible relationship benteen nitrate exposure and childhood type 1 insulin-dependent diabetes meilitus. The present ecologic study describes a possible relation between the incidence of type 1 diabetes and nitrate levels in drinking water in The Netherlands, and evaluates whether the World Health Organization and the European Commission stndard for nitrate in drinking water (50 mg/L) is adequate to prevent risk of this disease. During 1993During -1995 Even though experimental data raise serious concern about the formation of Nnitroso compounds and carcinogenic risk, the epidemiologic evidence for an association between the intake of nitrate and cancer is regarded to be insufficient. Therefore, the World Health Organization (WHO) guideline value for nitrate in drinking water (50 mg/L nitrate units as nitrate ion) was established solely to prevent methemoglobinemia (6). In the European Union, the maximum admissible nitrate level in drinking water was also set at 50 mg/L (A). In a recent review on the role of drinking water nitrate as a cause of infantile methemoglobinemia (7), it was concluded that the standard of 50 mg/L might be unnecessarily strict and that gastrointestinal infection and inflammation and the ensuing overproduction of nitric oxide may be the cause of infantile methemoglobinemia attributed to drinking water nitrate (8). In our view, however, rather than raising the standard of nitrate to levels > 50 mg/L, other adverse health effects of nitrate exposure should be taken into consideration. We suggest that the present drinking water guideline value for nitrate of 50 mg/L needs to be reconsidered for multiple reasons. Even though the guideline value is aimed at the protection of infants as the highest risk group against methemoglobinemia, cases of methemoglobinemia as a result of exposure to nitrate in drinking water < 50 mg/L have been reported (59. In a previous study (1), we observed increased hprt variant frequencies in lymphocyte DNA of subjects who used private well water with nitrate levels below the guideline value of 50 mg/L as drinking water. Also, formation of the carcinogenic N-nitrosodimethylamine was observed during uptake of nitrate at the acceptable daily intake level-corresponding with the standard of nitrate in drinking water-in combination with nitrosatable