An association has recently been described between increased birth weight and increased risk of childhood-onset type 1 diabetes. Whether this relationship is explained by genes associated with both increased birth weight and increased risk of type 1 diabetes is unknown. In the present study, we tested the association between birth weight and HLA-DQ genotypes known to confer risk for type 1 diabetes among 969 nondiabetic children randomly selected from the Norwegian population. We found that HLA genotypes previously shown to confer risk for type 1 diabetes were associated with reduced birth weight (the mean difference in birth weight between the DQB1*0602/DQB1*0602 and DQ8/DQ2 genotypes was 354 g [95% CI 105-604]), which was opposite of that expected if HLA genes explained the birth weight-type 1 diabetes association.