Pycnodysostosis is a term introduced by Maroteaux and Lamy in 1962 to describe characteristic features such as dwarfism, separated cranial sutures, open fontanelles, hypoplasia and absence of mandibular angle, partial aplasia of the terminal phalanges of the fingers and toes, generalized condensation of bone shadow, bone fragility, and autosomal recessive inheritance. We reported a such case in a 13-years old boy associated with an epidural hematoma. Four weeks after removal of the hematoma, lumbar puncture was performed with the initial pressure of 200mm H2O in an awake state and the plateau waves were observed several times during sleep. Such increased intracranial pressure may have caused the open fontanelle and cranial sutures in addition to dysostosis. Thus such patients require careful attention in order to prevent major bleeding from the superior sagittal sinus directly beneath the separated sagittal suture.