The authors report a case of a preterm infant at 29 weeks of gestation who gradually developed a Chiari malformation Type I (CM-I) with hydrocephalus due to a premature unilateral fusion of the posterior intraoccipital synchondrosis. Brain ultrasonography results in the 1st week of life were normal. Follow-up ultrasonography showed progressive development of triventricular hydrocephalus. Brain MRI demonstrated the presence of a CM-I and a deformation of the occipital bone. A complementary CT scan was obtained, showing a closure of the right posterior intraoccipital synchondrosis, resulting in a deformation of the posterior cranial fossa. This case shows the close relationship between a malformation of the skull base and the secondary development of a brain malformation. The authors discuss the anatomy of the occipital bone and suggest a probable theory for the premature closure of this synchondrosis and the consequent development of a CM-I. The originality of this case lies in the observation of the natural history of a brain malformation in a preterm infant.