Intrauterine constraint is responsible for a number of structural defects of the craniofacial and peripheral skeleton including plagiocephaly, micrognathia, congenital hip dislocation, and talipes equinovarus. This report describes five infants with serious craniofacial alterations, all attributable to intrauterine constraint. All five children had clinical and laboratory evidence strongly suggestive of craniosynostosis. In three of the five, sutural fusion was documented either at operation or at necropsy. In the other two the clinical course indicated that true synostosis was not present. In all cases the sutural involvement corresponded to the dimension in which head growth had been restricted. This determaination was based upon an assessment of the other craniofacial alterations that were present. The etiology of the intrauterine constrain was different in each case. Factors involved included breech presentation, primagravidity, uterine malformations, amniotic bands, and defects in fetal neuromuscular development, all of which are known to produce fetal deformations. As further evidence of severe constraint, fetal activity was greatly reduced during four of the five pregnancies. There was no history of craniosynostosis in other family members. We propose that in utero compression may lead to a spectrum of craniofacial defects that includes craniosynostosis when the constraint is particularly prolonged. The data suggest that mechanical forces may play a role in the etiology of some cases of craniosynostosis. The fact that head shape spontaneously remolded in one of the five cases suggests further that surgical correction may not be required in all cases in which constraint is the suspected etiology of the deformation. (Neurosurgery, 6: 39-44, 1980).