Occipitofrontal head circumference was measured on 122 twin pairs, their spouses, and their children. These data indicate that approximately 50% of normal head size variation is familial. Because of the relationship between the head size of normal children and their parents, adjustment of a child's head size value by the average parental value permits better definition of the range of normalcy. A method is presented that will allow physicians to make this adjustment, providing a more refined assessment of head size when there is a suspected abnormality.