Type 1 diabetes mellitus is recognized as a T-cell-mediated autoimmune disease. Vitamin D compounds are known to suppress T-cell activation by binding to the vitamin D receptor (VDR); and thus, VDR gene polymorphisms may be related to T-cell-mediated autoimmune diseases. We, therefore, investigated a VDR gene polymorphism in type 1 diabetes. We examined the VDR gene Bsm I polymorphism in 203 type 1 diabetic patients and 222 controls, and the association between the VDR gene polymorphism and type 1 diabetes and their onset pattern. We found a significantly higher frequency of B allele in type 1 diabetics overall, compared with controls (P = 0.0010). Moreover, there was a significant difference in B-allele frequency between acute-onset type 1 diabetics and controls (P = 0.0002), whereas this difference was not observed between slow-onset type 1 diabetics and controls. Regardless of the existence of islet-associated autoantibody, we found a significant difference in B-allele frequency between acute-onset type 1 diabetics and controls. In conclusion, we found an association between a VDR gene polymorphism and acute-onset type 1 diabetes. Assessment of this VDR gene polymorphism may contribute to prediction of the onset pattern in individuals with a high risk of type 1 diabetes.