The incidence of type 2 diabetes is increasing among Finnish young adults. A slightly increased risk in men was found in the north-east and western part of the country. The higher risk areas in women were found in the western coastal area and in eastern Finland. The present register-based study aimed to evaluate the regional association of the incidence of type 2 diabetes among young adults with the concentration of magnesium in local ground water. The association was evaluated using Bayesian modeling of geo-referenced data aggregated into a regular 10 km x 10 km grid cells. No marked association was found, although suggestive findings were detected for magnesium in well water and diabetes in young adult women. The results of this register-based study did not completely rule out the association of well water magnesium with the geographical variation of type 2 diabetes. The incidence of type 2 diabetes was much higher among individuals aged 40 or over. These suggestive findings indicate that the association between magnesium and type 2 diabetes would also be worth examining among individuals over 40 years of age. (C) 2011 Elsevier Inc. All rights reserved.