Three cases of syringomyelia associated with arachnoid webs are reported. Each patient presented with progressive myelopathy and had thoracic syringes detected on magnetic resonance imaging (MRI). In one patient the web was also visible. At operation a thoracic arachnoid web was found, obstructing the subarachnoid compartment in each patient. One patient had intraoperative ultrasound, which demonstrated caudal web movement with each cardiac systole. The webs were divided and shunts inserted into the syringes. All patients improved clinically, and on follow-up MRI. Arachnoid webs are likely to represent a focal band of arachnoiditis and are difficult to visualise on standard preoperative MR imaging. A reduction in the subarachnoid space compliance with resultant increase in pulse pressure and potentiation of an arterial pulsation driven perivascular flow could explain the associated syringes. Treatment should be aimed at restoring compliance, and involve division of the web with or without shunt insertion.