Two children who presented with rapidly worsening neurological symptoms attributable to a previously undiagnosed Chiari malformation Type 1 are reported from Children's Hospital, Birmingham, AL. One patient became hypopneic and dysphagic and developed a right hemiparesis in less than a 48-hour period. Another patient presented with a rapidly worsening right hemiparesis, ataxia, and anisocoria. MRI revealed the Chiari 1 in both patients, and a syrinx was also identified in the second patient. Following surgical posterior fossa decompression, symptoms immediately improved. 
COMMENT. Acute presentation of Chiari malformation Type 1 (CM-1) is rare, especially in children. Chiari 1 should be included in the differential diagnosis of acute onset of brainstem or long tract signs. In a study of CM-1 at the Children's Hospital, Birmingham, UK, abnormalities of the skull base were identified by MRI measurements, indicative of a mesodermal defect.