Objective-Associations between single-nucleotide polymorphisms in the β2-adrenergic receptor gene and asthma and wheeze have been inconsistent. Recent studies indicated that tobacco smoke affects β2-adrenergic receptor gene expression and associations of β2-adrenergic receptor gene variants with asthma in adults. We aimed to investigate the joint effects of in utero and childhood secondhand tobacco smoke exposure and 2 well-characterized functional single-nucleotide polymorphisms (Arg16Gly and Glu27Gln) of β2-adrenergic receptor gene on asthma and wheezing in 3128 non-Hispanic and Hispanic white children of the Children's Health Study.Methods-We fitted logistic regression models to estimate odds ratios and 95% confidence intervals for the independent and joint effects of these single-nucleotide polymorphisms and in utero and secondhand tobacco smoke exposure on asthma and wheeze outcomes.Results-Exposures to in utero maternal smoking and secondhand tobacco smoke were associated with wheezing. Children who were homozygous for the Arg16 allele and were exposed to maternal smoking in utero were at a threefold increased risk for lifetime wheeze compared with children who were unexposed and had at least 1 Gly16 allele. We found similar joint effects of secondhand tobacco smoke and Arg16Gly with wheezing. The risk for lifetime, current, and nocturnal wheeze increased with the number of smokers at home among Arg16 homozygous children. The results were consistent in 2 cohorts of children recruited in 1993 and 1996. Diplotype-based analyses were consistent with the single-nucleotide polymorphism-specific results. No associations were found for Glu27Gln.Conclusions-Both in utero and childhood exposure to tobacco smoke were associated with an increased risk for wheeze in children, and the risks were greater for children with the Arg16Arg genotype or 2 copies of the Arg16-Gln27 diplotype. Exposures to smoking need to be taken into account when evaluating the effects of β2-adrenergic receptor gene variants on respiratory health outcomes. In ADRB2, Arg16Gly and Glu27Gln are the 2 most common nonsynonymous single-nucleotide polymorphisms (SNPs). In an in vitro study, Green et al 1 found that the Gly16 allele was more susceptible to agonist-promoted downregulation, and the Glu27 allele was resistant to agonistmediated receptor downregulation; however, subsequent ex vivo and in vivo studies provided inconsistent findings. 2 The majority of the epidemiologic studies have investigated the associations of these 2 SNPs and asthma/wheeze and found inconsistent results. In addition, meta-analyses that pooled data from these studies failed to show any significant associations with asthma; however, these authors could not rule out effects of these SNPs on asthma symptoms. [3][4][5][6] The inconsistencies across studies may have resulted from not taking into account environmental factors that could modulate the receptor activity. Tobacco smoke is 1 such factor that is associated with an increase in AHR and oxidant stress-mediated...