Sagittal craniosynostosis is the most common of all craniosynostoses. Patients with sagittal craniosynostosis exhibit a typical cranial appearance, including scaphocephaly, and an elongated head, with a fused and ridged sagittal suture. Moreover, some recent reports described atypical sagittal craniosynostosis accompanied by autism, speech delay, and hyperactivity. This condition, known as delayed-onset craniosynostosis, is rarely reported, given that it is difficult to determine if the condition is congenital or has a delayed-onset.This report describes the clinical course and treatment of 2 brothers with atypical sagittal synostosis. The shapes of their heads were not indicative of scaphocephaly and the younger brother exhibited delayed-onset sagittal synostosis. Their father and paternal grandmother exhibited similar cranial morphologies. Therefore, we hypothesized the involvement of a familial factor in the etiology of atypical sagittal synostosis in these patients.