We screened the 5'-untranslated region of the beta2-adrenergic receptor gene from 40 obese subjects by the PCR-direct sequencing technique. Two polymorphic sites were identified; a T-->C substitution at -47 and a T-->C substitution at -20. We further analyzed the association of the polymorphisms with obesity in 574 subjects by PCR and restriction digestion. The substitution at -47 was in tight linkage disequilibrium with that at -20. The polymorphisms were also in linkage disequilibrium with codon 16 and codon 27 polymorphisms. Subjects carrying the -47C/-20C allele had greater body mass index (25.5+/-4.5 vs. 24.4+/-4.1 kg/m2, p=0.007) and higher serum triglyceride levels (166+/-160 vs. 139+/-95 mg/dl, p=0.015) than -47T/-20T homozygotes. The variant allele frequency was significantly higher in obese subjects than in non-obese subjects (0.18 vs. 0.11, p=0.0026). Furthermore, an increased frequency of the variant allele was shown in diabetic patients compared with non-diabetic subjects (0.19 vs. 0.11, p=0.0005). The association may be attributable to the greater proportion of diabetic patients in the obese group. The exchange at -47 may alter the expression level of the beta2-adrenergic receptor gene, because the nucleotide substitution at -47 results in a Cys-->Arg exchange at the C terminal of the leader peptide. The -47C/-20C allele may be associated with genetic predisposition to obesity and obesity-related metabolic disorders.