Multiple-suture synostosis is typically associated with syndromic craniosynostosis but has been occasionally reported in large series of nonsyndromic children. The diagnosis of multiple fused sutures usually occurs at the same time, but rarely has the chronological development of a secondary suture synostosis been noted. The development of secondary bicoronal suture synostosis requiring surgical intervention has only been reported, to date, after surgical intervention and is hypothesized to arise from a disruption of inhibitory factors from the dura. The disinhibition of these factors permits the sutures to then fuse at an early stage. The authors report on a patient who developed secondary unilateral coronal synostosis after the diagnosis of an isolated sagittal synostosis. The secondary synostosis was identified at the time of the initial surgical intervention and ultimately required a second procedure of a frontoorbital advancement. The clinical appearance of this phenomenon may be subtle, and surgeons should monitor for the presence of secondary synostosis during surgery as it may require intervention. Failure to identify the secondary synostosis may necessitate another surgery or result in a poor cosmetic outcome. The authors recommend close clinical follow-up for the short term in patients with isolated sagittal synostosis.