138 small-for-dates (SFD), 138 average-for-dates (AFD) and 136 large-for-dates (LFD) children who had been followed up from birth were examined at the age of 7 years. Nine children had major congenital abnormalities (SFD 5, AFD 2, LFD 2). In addition gross and/or fine motor incoordination was noted in 25 children (SFD 9, AFD 10, LFD 6). There were no differences between the groups in the incidence of chronic or specific ill-health, hearing, sight and speech defects. Highly significant differences were found between the groups for weight, height, head circumference and triceps skinfold thickness; and significant differences were also found between the total developmental scores in the three groups. In every case SFD children had the lowest and LFD children the highest mean values. Within each group analyses were made of 22 parental, pregnancy, perinatal and postnatal factors which might affect growth or development. The net effects of those factors for which significant differences had been found were then assessed, adjustment being made for confounding between variables. In all three groups the children's genetic potential for size was strongly evident by this age, and boys had larger heads than girls. All three groups showed the powerful influence of social class on intellectual ability; and sex on gross and fine-motor function. The other main contributor to developmental differences in the AFD group was gestational age. In the SFD group maternal hypertension was associated with slightly decreased intellectual ability, and in all areas first-born SFD children performed better than subsequent born. Among LFD children instrumental delivery had an adverse effect on all areas of ability, and first-born children had higher intellectual scores. When all the children were considered together and birthweight included as an additional variable the differences in developmental scores between the groups were much reduced. In the SFD and LFD groups some significant correlations were found between size and developmental scores; but none were found among AFD children.