Although MHC class II genes have a stronger association with type 1 diabetes than MHC class I genes, studies have shown that MHC class I molecules play an independent role in the etiology of type 1 diabetes, and the existence of susceptibility genes within a segment of MHC between the HLA-B and TNF genes has been predicted, where MHC class I chain-related gene A (MICA) resides. MICA has a triplet repeat polymorphism in the transmembrane region consisting of five alleles. We analyzed this polymorphism in 162 unrelated children (82 boys) with type 1 diabetes (age at diagnosis 7.01 +/- 3.76 yr) and 154 randomly selected unrelated children (87 boys), age 2.81 +/- 2.12 yr. Phenotype frequency of allele A9 in children with type 1 diabetes was significantly higher than in controls (RR = 2.42, 95% CI = 1.52-3.85, p = 0.000162, pc = 0.00081). Gene frequency of allele A9 was also significantly higher in children with type 1 diabetes when compared with control children (RR = 2.73, 95% CI = 1.85-4.03, p = 2.62 x 10(-7), pc = 1.31 x 10(-6)). This study demonstrates that MICA allele A9 confers risk of type 1 diabetes.