tLow birth weight is associated with exposure to air pollution during pregnancy. The purpose of thisstudy was to evaluate whether null polymorphisms of Glutathione S-transferases (GSTs), specificallyGSTM1 and GSTT1 genes in infants or mothers, modify the association between high exposures to house-hold air pollution (HAP) from cooking fires and birth weight. Pregnant women in rural Guatemala wererandomized to receive a chimney stove or continue to use open fires for cooking. Newborns were mea-sured within 48 h of birth. 132 mother–infant pairs provided infant genotypes (n = 130) and/or maternalgenotypes (n = 116). Maternal null GSTM1 was associated with a 144 g (95% CI, −291, 1) and combinedmaternal/infant null GSTT1 was associated with a 155 g (95% CI, −303, −8) decrease in birth weight.Although there was a trend toward higher birth weights with increasing number of expressed GST genes,the effect modification by chimney stove use was not demonstrated