BACKGROUND:Type 1 diabetes (T1D) is an autoimmune disease which is characterised by the destruction of insulin-producing beta cells in human pancreas leading consequently to a hyperglycaemic metabolism. Recent studies have shown that low cholecalciferol (25(OH)D3) concentrations may contribute to the development of T1D. The 25(OH)D3 status depends mostly on human skin production influenced by exposure to UVB radiation. Our intention was to examine whether there was a change in UVB radiation in the past years and if this has an impact on patients' vitamin D status. METHODS:We analysed the 25(OH)D3 concentration of blood samples from 287 T1D patients in the years 2004-2007 at the University Hospital Frankfurt. Moreover, daily UVB irradiation data of this time were received. Wilcoxon-Mann-Whitney test and Spearman correlation test were used for statistical analyses. RESULTS:We observe a strong correlation between UVB irradiation and the 25(OH)D3 concentration of German T1D patients (correlation coefficient=rho=0.56, p=7×10(-3)). Moreover, 25(OH)D3 blood levels obtained in summer (Apr-Oct) were significantly higher than in the winter season (p=8×10(-3)). In the years 2004-2007 there was a significant decline of UVB radiation in the summers (rho=-0.21, p<10(-6)) but no change was found in (rho=-0.07, p=0.12). This corresponds to a significant decrease of 25(OH)D3 levels in T1D patients over the summers (rho=-0.24, p=2×10(-3)) but not in winters (rho=-0.03, p=0.73). CONCLUSION:Our results reveal a significant correlation of UVB irradiation and the vitamin D concentration of German T1D patients. A decrease of UVB irradiation over the summers 2004-2007 is accompanied by a decline of 25(OH)D3 levels observed in those summer months which may indicate a local time trend requiring further investigation into the environmental factors of vitamin D deficiency. This article is part of a Special Issue entitled 'Vitamin D Workshop'.