This study investigated the effects of smoking and alcohol consumption in pregnancy on length, head circumference, upper arm circumference and ponderal index, of neonates born to 1513 Caucasian women who delivered at St George's Hospital, south London. All measurements were adjusted for gestational age, maternal height, parity and sex of infant. Babies of smokers were shorter, had lower ponderal index and smaller upper arm circumference than those of non-smokers. After controlling for alcohol consumption, these differences remained (but with reduced statistical significance). There was no statistically significant difference in head circumference between smokers and non-smokers. Alcohol consumption at booking had no effect on growth measurements in non-smokers but had a significant, negative effect on all measurements in smokers. Drinking later in pregnancy had less effect. Alcohol appears to enhance the growth-retarding effect of smoking. It is suggested that both smoking and alcohol also have an inhibitory effect on fat deposition in babies, which contributes to the reduction in birthweight associated with smoking and drinking.