BACKGROUND: Isolated synostosis of the frontosphenoidal suture is very rare and difficult to diagnose. Little has been reported on the clinical presentation and fetal development of this suture. OBJECTIVE: To understand the development of the frontosphenoidal suture and the outcome of its synostosis. MATERIALS AND METHODS: We studied the normal fetal development of the frontosphenoidal suture in dry human skulls and the clinical features of four patients with isolated synostosis of the frontosphenoidal suture. RESULTS: The frontosphenoidal suture develops relatively late during the second trimester of pregnancy, which explains the mild phenotype when there is synostosis. This rare craniosynostosis results in a deformity that causes recession of the lateral part of the frontal bone and supraorbital rim, with minimal facial asymmetry. Three-dimensional CT is the best examination to confirm the diagnosis. CONCLUSION: Isolated frontosphenoidal synostosis should be considered in patients with unilateral flattening of the forehead at birth that does not improve within the first few months of life.