A missense mutation of the beta3-adrenergic receptor gene (Trp64Arg) has been associated with obesity and increased capacity to gain weight in nonpregnant populations. Furthermore, the mutation is a potential modifying factor in the etiology of impaired glucose tolerance and type 2 diabetes. We studied the relation of the beta3-adrenergic receptor genotype to glucose tolerance during pregnancy, a state of physiological insulin resistance. In 179 pregnant women (mean age, 28.5 +/- 0.4 yr), a 2-h oral glucose tolerance test was performed between gestational weeks 20 and 31. The beta3-adrenergic receptor genotype was assessed using restriction fragment length polymorphism. The frequency of the Arg64 allele was 9.15%. In women with mild gestational diabetes (n = 70), as defined by 60 min postload glucose values, the Trp64Arg genotype was more frequent than in women with normal glucose tolerance (n = 109; 26% vs. 11%; P = 0.01). Furthermore, the Trp64Arg polymorphism was associated with increased weight gain during pregnancy (baseline to gestational weeks 20-31) and increased postload glucose, insulin, and C peptide values during the oral glucose tolerance test. The results of the present study extend current knowledge about the association of the Trp64Arg beta3-adrenergic receptor polymorphism with glucose tolerance to a pregnant population. The association with mild gestational diabetes suggests that the impact of the polymorphism may be clinically important during pregnancy, a state of physiological insulin resistance.