OBJECT: The effects of posterior fossa decompression on Chiari malformation Type I-induced syringomyelia have been well described. However, treatment of worsening syringomyelia after Chiari decompression remains enigmatic. This paper defines patient and clinical characteristics as well as treatment and postoperative radiological and clinical outcomes in patients experiencing this complication. METHODS: The authors performed a retrospective review of patients at the Children's Hospital of Pittsburgh and Children's of Alabama who developed worsening syringomyelia after Chiari decompression was performed. RESULTS: Fourteen children (age range 8 months to 15 years), 7 of whom had preoperative syringomyelia, underwent posterior fossa decompression. Aseptic meningitis (n = 3) and bacterial meningitis (n = 2) complicated 5 cases (4 of these patients were originally treated at outside hospitals). Worsening syringomyelia presented a median of 1.4 years (range 0.2-10.3 years) after the primary decompression. Ten children presented with new, recurrent, or persistent symptoms, and 4 were asymptomatic. Secondary Chiari decompression was performed in 11 of the 14 children. The other 3 children were advised to undergo secondary decompression. A structural cause for each failed primary Chiari decompression (for example, extensive scarring, suture in the obex, arachnoid web, residual posterior arch of C-1, and no duraplasty) was identified at the secondary operation. After secondary decompression, 8 patients' symptoms completely resolved, 1 patient's condition stabilized, and 2 patients remained asymptomatic. Radiologically, 10 of the 11 children had a decrease in the size of their syringes, and 1 child experienced no change (but improved clinically). The median follow-up from initial Chiari decompression was 3.1 years (range 0.8-14.1 years) and from secondary decompression, 1.3 years (range 0.3-4.5 years). No patient underwent syringopleural shunting or other nonposterior fossa treatment for syringomyelia. CONCLUSIONS: Based on the authors' experience, children with worsening syringomyelia after decompression for Chiari malformation Type I generally have a surgically remediable structural etiology, and secondary exploration and decompression should be considered.