The prevalence of type 2 diabetes is increasing, which for most societies has considerable consequences not only regarding health but also economy. Type 2 diabetes develops through a "prediabetic" stage with impaired glucose tolerance. Intervention at this stage with change in lifestyle or with medication may prevent such progression. There are indications that vitamin D is of importance in glucose metabolism, and that supplementation with vitamin D may increase both insulin secretion and insulin sensitivity. Accordingly, supplementation with vitamin D may improve glucose tolerance and potentially prevent the development of type 2 diabetes in subjects at risk. However, this has so far not been demonstrated in a prospective, randomised clinical study. In the present study we will therefore include 600 subjects with impaired glucose tolerance (or impaired fasting glucose) detected in the Tromso study 2007/2008 and randomize to supplementation with vitamin D 20.000IU per week or placebo for 5 years. A glucose tolerance test will be performed each year, and development of type 2 diabetes will be the main endpoint.