From a series of 1,408 patients with craniosynostosis hospitalized between 1976 and 1994, 561 probands with non-syndromal isolated sagittal synostosis were analyzed. The prevalence of sagittal synostosis was estimated in the order of 1 in 5,000 children. Family information was obtained from 373 probands distributed among 366 families. The male:female ratio was 3.5:1. There was no maternal or paternal age effect. In 22 of the 366 pedigrees, a high degree of familial aggregation was observed, giving a 6% figure of familial cases. Segregation analysis of 253 families indicates that sagittal synostosis is transmitted as a dominant disorder with 38% penetrance and 72% of sporadic cases. The frequency of twinning was 4.8% with only 1 concordance for sagittal synostosis in a monozygotic twin pair. The possibility of a mechanical pathogenesis in sporadic cases is discussed.