OBJECT: The incidence of syringomyelia in patients with Chiari I malformation is reported to be between 20 and 75%. In the series reported herein 10.6% of patients with both clinical entities have continued to have a significant syrinx following their first decompressive procedure. All but one patient had resolution of their syrinx after a second posterior fossa operation. The authors have analyzed this smaller group of patients to look for possible radiological or surgical findings that may aid in predicting which patients are less likely to respond to the initial first decompressive procedure. METHODS: The authors retrospectively reviewed radiological and operative data in eight patients who continued to have syringomyelia following a decompressive procedure. Seven (88%) of these patients had complete resolution of their syrinx following a second operation. At repeated operation, obstruction at the foramen of Magendie was seen in six patients. In one patient in whom the dura was not opened during first operation, the second operation revealed an arachnoid veil that occluded the foramen of Magendie. CONCLUSIONS: No single radiological measurement was found to aid in the prediction of which patients would not respond to the first decompressive procedure. Furthermore, no operative finding was extraordinarily unique to any single patient. All but one patient in whom confirmation of a patent foramen of Magendie was made at repeated operation-that is, lysing of arachnoid veils, stent placement, unilateral tonsillar coagulation-had resolution of their syringomyelia. Surgical reexploration should be considered in cases of persistent syringomyelia.