Fifteen normal children with large heads (circumference greater than 0.5 cm above the 98th centile) were studied. CAT scans were pefrormed to exclude hydrocephalus, and ventricular size was compared with that of hydrocephalic children. In 11 of the 13 families in which the parents' heads were measured, one parent (10 fathers and one mother) was found to have a large head, as had 6 of 17 siblings. Head circumference at birth was large in 7 of 10 babies and rate of head growth was excessive in 8 of 13. Skull x-ray showed suture diastasis in 7 infants. These families have a benign familial megalencephaly. It is important to recognise this so as to avoid unnecessary investigation and anxiety about normal children with large heads.