The authors report the case of a girl with achondroplasia suffering from a progressively worsening hypotonic quadriparesis. CT scan showed slight dilatation of ventricular and subarachnoid spaces, with well-defined evidence of cortical sulci and gyri. This aspect was compatible with the diagnosis of macrocrania and megalencephaly (CP being 51 cm). The foramen magnum was narrowed, the transverse diameter measuring 15 mm and the 50th percentile being, for age, 26 mm. Somatosensory evoked potentials (SEPs) revealed bilaterally prolonged interpeak latencies Erb-N13, slowing of central conduction time N13-N20 from right median nerve stimulation, and block from left median nerve. The suspicion of cervicomedullary compression was confirmed by MRI, showing a very marked stenosis with compression exerted by the odontoid process. Further, a stenotic cervical canal and optic nerves verticalization were manifest. The patient underwent neurosurgical decompression by suboccipital craniectomy and cervical-C1 laminectomy. In spite of treatment, both neurologic and respiratory problems (rapid, shallow and almost abdominal breathing) were unchanged. The girl died 4 1/2 months later. The authors emphasize the important role of SEPs in detection of cervicomedullary compression in achondroplastic children and also stress the necessity of an early surgical treatment as the only condition for possible clinical improvement and/or full recovery.