There is scanty data in the literature on the variation of fontanelle size with gestational age. This relationship was studied in 250 neonates delivered at gestational ages of 29-41 weeks at the University College Hospital, Ibadan, Nigeria. Anterior fontanelle size showed a low positive correlation with gestational age (r = 0.15). However, controlling for occipitofrontal circumference reduced this correlation to 0.12. Mean anterior fontanelle size increased with increasing gestational age but the anterior fontanelle size: occipitofrontal circumference ratio remained remarkably constant over the range of gestational age studied. The posterior fontanelle size did not show a significant correlation with gestational age and the prevalence of closed posterior fontanelles at birth did not differ significantly between term and preterm neonates. It is concluded that anterior fontanelle size does show a low positive correlation with gestational age but maintains the same ratio with head circumference at least during the third trimester of intrauterine life. Posterior fontanelle size shows no linear relationship with gestational age.