We report a child with Costello syndrome and Chiari I malformation. The medical literature has several case reports of findings peculiar for each of these two clinical entities that, when investigated, can actually be found in both processes. Recent reports have shown additional medical coincidences for the Chiari I malformation, which, unlike the Chiari II malformation, was once thought not to have many additional associations. We propose that both Costello syndrome and other clinical entities that have potential mesodermal or ectodermal deficiencies (eg, Chiari I malformation and phakomatoses, respectively) could have common dysgeneses. Further case reports from other institutions regarding Costello syndrome and Chiari I malformation are now necessary to confirm our findings. Our hope is that these data will potentially add to our knowledge of the etiology of both Costello syndrome and Chiari I malformation and potentially aid in the definition of a genetic locus for both entities.