BACKGROUND Approximately 30% of patients treated with foramen magnum decompression (FMD) for Chiari I-associated syringomyelia will show persistence, recurrence, or progression of the syrinx. OBJECTIVE This study evaluates the clinical and radiologic outcomes of syringo-subarachnoid shunt (SSS) as the treatment for persistent syringomyelia after FMD. METHODS Data were collected retrospectively. The primary outcome measurement was neurologic function (assessed with the Modified Japanese Orthopedic Association [mJOA] scale). Secondary outcome measurements were surgical complications, reoperation rate, and syrinx status on magnetic resonance imaging (MRI). RESULTS Twenty-one patients (14 females [66.7%]) underwent SSS, either concurrent to the FMD or at a later stage. Two minor surgical complications were seen: a wound dehiscence and postoperative kyphosis, both requiring revision surgery. No major complication or mortality occurred. The median change in the mJOA score was an improvement of 3 out of a possible 17 points on the scale (mean follow-up, 24.9 months). Expressed as a percentage, overall improvement was 11.8% (95% confidence interval [CI], 5.9-17.6; P < 0.001). On postoperative MRI, shrinkage of the syrinx was seen in all but 1 patient in whom the syrinx remained unchanged. Expressed as percentage, the improvement of the syrinx surface was 76.3% (95% CI, 65.0-87.7; P < 0.001), and the improvement of syrinx span was 36.4% (95% CI, 21.8-50.9; P = 0.05). CONCLUSIONS SSS for persistent, recurrent, or increasing syrinx following FMD for Chiari I malformation is a safe and effective surgical treatment when performed selectively by an experienced neurosurgeon.