The origins of both sagittal synostosis (scaphocephaly) and metopic synostosis (trigonocephaly) remain unclear. Genetic and environmental factors probably play a role. Twin and family data of patients with these conditions are presented. Records of 860 patients with scaphocephaly and 394 with trigonocephaly were examined for prevalence of twin birth and family history. Concordance rates of disease in monozygotic (MZ) and dizygotic (DZ) twins were compared. A family history of disease was present in 5.7% of children with scaphocephaly and in 6.8% with trigonocephaly. The frequency of twins was 4.2% in sagittal synostosis and 6.8% in metopic synostosis, which is higher than in the normal population. Twenty-eight of 38 twin pairs with a scaphocephalic proband were identified to be DZ and 10 were confirmed to be MZ (MZ/DZ ratio 0.36). Twenty of 27 twin pairs with trigonocephaly were DZ and seven were MZ (MZ/DZ ratio 0.35). Concordance rates were higher in the MZ group, 30% for scaphocephaly and 43% for trigonocephaly (chi2 = 14.4091, p = 0.0001). There was a strong prevalence of boys, with a frequency of 79.1% with scaphocephaly and 75.8% with trigonocephaly. Twinning was more frequent among affected boys in both groups. A midline craniosynostosis is more likely to develop in twins compared with singletons. A genetic component is supported by the higher risk in MZ twins. The presence of an environmental component is reinforced by the high rate of twinning, the normal MZ/DZ ratio, and a less than 100% concordance rate in MZ twins.