Achondroplasia is the most common of the heritable skeletal dysplasias. Compression at the cervicomedullary junction can result in myelopathy, hypotonia, sleep apnea, and even sudden death. However, most children with achondroplasia do not suffer from severe neurological symptoms and achieve normal motor and intellectual development without surgical intervention. At the authors' institution, magnetic resonance (MR) imaging and cerebrospinal fluid (CSF) flow studies have been incorporated in the assessment of children with achondroplasia for cervicomedullary junction compression. The authors recently identified four children with achondroplasia who had normal findings on MR imaging and flow studies obtained in the neutral position. On flexion studies, however, three had complete blockage of CSF flow, and more dramatic posterior cervicomedullary compression was demonstrated on extension studies. Some of these patients had severe neurological abnormalities and sleep apnea, while others just developed headaches and/or had apnea episodes when sleeping or in a car seat. Three children underwent decompressive surgery with dramatic improvement or resolution of signs and symptoms. The fourth patient had increased CSF pressure on MR images obtained in the flexed position, possibly due to venous outflow obstruction. Her condition improved dramatically after placement of a ventriculoperitoneal shunt. The increased risk of dynamic cord compression and alterations in CSF dynamics in patients with achondroplasia constitute indications for surgical intervention.