Context: The β2-adrenergic receptor (ADRB2) influences regulation of energy balance by stimulating catecholamine-induced lipolysis in adipose tissue. The rare functional ADRB2rs1800888(Thr164Ile) polymorphism could therefore influence risk of obesity and subsequently diabetes. Objective: We tested the hypothesis that the ADRB2rs1800888(Thr164Ile) polymorphism associates with risk of obesity and diabetes and compared effect sizes with those of FTO(rs9939609), MC4R (rs17782313), and TMEM18(rs6548238). Design and Setting: We conducted a population-based cohort study in Copenhagen, Denmark. Participants: We genotyped more than 64,000 individuals from the Danish general population. Main Outcome Measures: We evaluated body mass index (BMI), obesity (BMI≥30 kg/m2), and diabetes. Results: Rare allele frequencies were 0.02 for T for ADRB2rs1800888(Thr164Ile), 0.40 for A for FTOrs9939609, 0.25 for C for MC4Rrs17782313, and 0.20 for T for TMEM18rs6548238. For rare vs. common homozygotes, odds ratio for obesity was 3.32 (95% confidence interval = 1.08-10.19) for ADRB2rs1800888(Thr164Ile), 1.42 (1.35-1.52) for FTOrs9939609, 1.18 (1.06 -1.30) for MC4R rs17782313, and 1.28 (1.10 -1.50) for TMEM18rs6548238 (common vs. rare). Corresponding odds ratios for diabetes were 1.85 (0.24 -14.29), 1.22 (1.07-1.39), 0.96 (0.80 -1.16), and 1.61 (1.17-2.22), respectively. After adjustment for BMI, only TMEM18rs6548238 remained associated with diabetes. BMI was increased in rare vs. common homozygotes in FTOrs9939609, MC4Rrs17782313, and TMEM18rs6548238 (common vs. rare) but not in ADRB2rs1800888(Thr164Ile). Conclusions: Our results suggest that ADRB2rs1800888(Thr164Ile) rare vs.common homozygotes are not significantly associated with an increase in BMI measured continuously but may be associated with an increased risk of obesity. Also,TMEM18rs6548238 associated with risk of diabetes after adjustment for BMI. These findings need confirmation in other studies.