Insulin-like growth factor 1 (IGF1) plays an important role in the development and function of pancreatic β-cells and contributes to infant growth, which we recently reported to be associated with type 1 diabetes (T1D). Here, we studied an IGF1 microsatellite in 206 families with T1D and its interaction with the polymorphism near the insulin (INS) gene variable number of tandem repeats. The IGF1 microsatellite was associated with T1D (P = 0.045), which was mainly caused by a protective effect of the 194 bp allele (36% transmission to affected offspring). Interestingly, co-segregation of this IGF1 194 bp allele affected the risk of INS alleles. These results provide the first evidence for an association of IGF1 with T1D and imply that co-inheritance of these functional genetic variants of IGF1 and insulin predispose to T1D.