The Brazelton Neonatal Behavioural Assessment Scale was used to compare the behaviour patterns of infants of mothers who smoked during pregnancy and those whose mothers did not smoke. Smokers were mothers who had smoked more than 15 cigarettes/day throughout their pregnancy. Over a period of 9 mth, a total of 32 infants (15 of whose mothers smoked and 17 whose mothers did not smoke) between 4 and 6 days of age were examined and the results compared. The two groups were matched for maternal age, social class and parity. All infants were spontaneously delivered at term and of normal birthweight. Sex distribution was equal. Duration of labour and analgesia during labour were similar for smokers and non-smokers. Examinations were carried out in a warm, quiet semi-dark room, the examiner being unaware of which category the infant was in. There was evidence to suggest that the behavioural patterns of infants can be influenced by smoking in pregnancy and that in particular the auditory senses are affected.