The cranial size and configuration of the Apert cranium are unique. True megalencephaly is characteristic, postmortem brain weights being dramatically increased above the 95th centile regardless of age. In addition, all Apert newborn infants have coronal synostosis with a widely patent midline calvarial defect. Thus, the head is unusually heavy and the cranium is disproportionately high. These characteristics, which are present at birth, result in a mean newborn length and weight above the 50th centile. The widely patent midline calvarial defect, allowing the brain to expand anteriorly into the metopic area, and some increase in the head breadth permit the mean head circumference at birth to normalize slightly above the 50th centile. In our series of surgically unoperated patients of different ages from the 1960s and earlier, most head circumference values fall below the mean but within or at -2 SD. Thus, the natural history of the unoperated growing cranium, beginning slightly above the 50th centile at birth, consists of a slowing of head circumference expansion to a greater degree than normal. Studies of intracranial volume show that mean adult male and female volumes far exceed normal adult values. Cranial shape is distinctive with head breadth either being normal or slightly increased, head length being significantly shortened, and head height being dramatically increased. The mean cephalic index is hyperbrachycephalic. Sexual dimorphism is found, with higher values in females than in males; the cause is unknown, although the contributing component appears to be head breadth. The disproportionately high cranium in the Apert syndrome is dramatically shown by the great differences from normal in the head height/head breadth index and in the head height/head length index. Finally, the crania of Apert and Crouzon syndromes are compared in terms of size, shape, and volume. For Crouzon syndrome, the mean adult cephalic index is normocephalic and the mean adult intracranial volume is smaller than normal.