• Children with achondroplasia may be at increased risk of developing apneic episodes and of dying unexpectedly. The risks seem to be related to neural axis compression by an abnormal cranial base and may be complicated by the development of hydrocephalus. We used magnetic resonance imaging to study five children with achondroplasia. All of them demonstrated a discrepancy between the size of the brain stem and the foramen magnum. Comprehensive prospective assessment of infants with achondroplasia, including the use of new imaging techniques, will provide important information concerning the natural history of the relationship of the neural axis to the bony posterior fossa and upper cervical spine in this condition. It may also help to identify those patients at risk before the development of life-threatening medullary compression.