The case of a 3-year-old patient with tuberous sclerosis and a 13-mm Chiari malformation Type I that spontaneously disappeared over the course of 4 years is presented. Using morphometric measurements of the posterior fossa and cerebellum in this patient, the authors show that the volume of the posterior fossa at the time of initial evaluation was consistent with that reported as normal in the literature (180.24 cm3; normal volume 132-198 cm3). Moreover, the patient showed a normal rate of growth of his posterior fossa over the period of observation (201.05 cm3; normal range 153-230 cm3). Cerebellar volumes were found to increase only minimally during this time period, which is compatible with observations in healthy controls. The posterior fossa volume, on the other hand, was shown to increase significantly more than that of the cerebellum (p=0.0185). This differential growth may permit the tonsils to ascend back up into the posterior fossa. Therefore, pediatric patients with normal posterior fossa volumes and normal development may have a spontaneous resolution of their asymptomatic Chiari malformation Type I.