SummaryRationale Previous data have suggested that glutathione‐S‐transferase (GST) genotypes are important in determining the rate of lung function growth in childhood. This effect was most marked in Caucasian children with asthma.Objectives We investigated the association of lung function with GSTM1, GSTP1 and GSTT1 genotypes in Caucasian families with asthma.Methods Four hundred and eighteen children and 316 parents from 224 Caucasian families were recruited via a child with asthma, the proband. Associations between lung function and GST genotype were determined using multilevel models.Results There were no observed associations between lung function and GST genotype in parents. However, in the children, the GSTP1 val105/val105 and GSTM1 null genotypes were associated with significantly higher forced expiratory volume in 1 s (FEV1) and FVC values as percentage of predicted. This effect was not statistically significant in the probands but was marked in their siblings in whom GSTP1 val105/val105 was associated with 9.4% higher FEV1 and 10.7% higher FVC (P=0.005 and 0.001, respectively). The GSTM1 null genotype was associated with a 6.7% higher FEV1 and 4.1% higher FVC (P=0.003 and 0.063, respectively). These effects remained significant after correcting for the confounders of individual atopic status, tobacco smoke exposure and familial aggregation of lung function values.Conclusions GSTM1 and GSTP1 genotypes are important determinants of lung function in childhood. The smaller differences seen in probands are predicted by a simple model in which more rapid decline in lung function is seen in these individuals.