This study is an analysis of the relationship between ultraviolet B (UVB) irradiance, the primary source of circulating vitamin D in humans, and age-standardised incidence rates of type 1 diabetes mellitus in children, according to region of the world. The association of UVB irradiance adjusted for cloud cover to incidence rates of type 1 diabetes in children aged <14 years during 1990–1994 in 51 regions worldwide was assessed using multiple regression. Incidence data were obtained from the Diabetes Mondial Project Group. Incidence rates were generally higher at higher latitudes (R 2 = 0.25, p < 0.001). According to multiple regression, UVB irradiance adjusted for cloud cover was inversely associated with incidence rates (p < 0.05), while per capita health expenditure (p < 0.004) was positively associated (overall R 2 = 0.42, p < 0.0001). An association was found between low UVB irradiance and high incidence rates of type 1 childhood diabetes after controlling for per capita health expenditure. Incidence rates of type 1 diabetes approached zero in regions worldwide with high UVB irradiance, adding new support to the concept of a role of vitamin D in reducing the risk of the disease.