Growth patterns of the cranial sutures were studied by bone scanning in 29 children, 13 of whom had craniosynostosis. Primary craniosynostosis was initiated by localized bony fusion across a suture, associated with an abnormal accumulation of bone-seeking radiopharmaceuticals. As the abnormal fusion process extended along the suture, there was a corresponding spread of radionuclide. With complete fusion, uptake diminished. In addition to calvarial scanning, three abnormal sutures were scanned following surgical excision, which allowed direct comparison in vivo and in vitro. Calvarial bone scanning is helpful in the early detection of primary craniosynostosis.