Achondroplasia, the most common heritable skeletal dysplasia, may result in abnormality at the craniocervical junction, which is a potentially lethal problem in a subset of young infants with this disorder. We evaluated and followed an unbiased and unselected consecutive series of infants with achondroplasia, to better document the occurrence, frequency, and clinical presentation of craniocervical abnormalities. Of 53 prospectively ascertained infants, 5 were judged to have sufficient craniocervical junction compression to require surgical decompression. Intraoperative observation always showed marked abnormality of the cervical spinal cord, and all operated-on children showed marked improvement of neurological function. The most frequent clinical abnormalities within this subset were those expected for high cervical myelopathy. The best predictors of need for suboccipital decompression included lower-limb hyperreflexia or clonus, on examination; central hypopnea demonstrated by polysomnography; and foramen magnum measures below the means for children with achondroplasia. Infants with achondroplasia are at risk for potentially lethal sequelae of craniocervical junction abnormalities; selective intervention can be life and health saving, but individuals at high risk will be identified only if all affected infants undergo comprehensive assessment in infancy.