The objectives of this study were to describe the global geographic variation of microvascular and 1 macrovascular complications in childhood onset type 1 diabetes (T1D) and to relate any such variation to diabetes care activities such as self blood glucose monitoring and intensive insulin therapy. The DiaComp study is a multinational (17 countries) cross-sectional study of complications in T1D (n=2,657). All participants were diagnosed at < 15 years of age and had a diabetes duration of 5-24 years when surveyed. Complications were assessed by self-report of physician diagnosis. Twenty-two centres in 17 countries achieved at least a 67% response rate and are included in the analyses. Central European centres exhibited high rates of retinopathy (Lithuania=31.6%, Romania=24.2%), laser treatment (Lithuania=25.4%) and neuropathy (Lithuania=29.9%, Romania=12.4%) in those with short duration of diabetes (5-15 years), as did Cuba for neuropathy (15.4%). For retinopathy the geographic variation in the short-duration group was also pronounced, ranging from 1.6% in Italy to 41.6% in Lithuania, and from 0% in Brazil, Italy and Australia, to 29.9% in Lithuania for laser treatment. Variation was less dramatic for the prevalence of complications in the long-duration group (15-25 years). Hypertension and duration were strong consistent predictors of all complications, while women had higher prevalence for half the complications (retinopathy, laser treatment and renal disease). Intensive insulin therapy and self-monitoring of blood glucose showed little association with prevalence of complications. In conclusion, this first population-based account of the geographic variation of T1D complications has demonstrated substantial variation. However, the healthcare practice variables that were measured contributed little toward explaining this variation.