Data on 365 children who had been small (SGA) average (AGA) or large (LGA) for gestational age at birth were analysed at the age of seven years. Significant positive correlations were found between head circumference and developmental scores in the SGA and LGA groups. These were mainly due to associations between relatively small heads and low scores. Familial factors, social class and sex affect both head circumference and developmental ability at this age. Their influences on these two parameters differ in degree; and in the case of sex, in direction. When account was taken of these other factors the contribution of birth-weight group to the variance in head circumference remained high; but its contribution to developmental scores was reduced.