A 25-year-old woman, who was 25 weeks pregnant, underwent insertion of a VP shunt for hydrocephalus, secondary to a bithalamic glioma. Two months later, she represented with symptoms of raised intracranial pressure and MR scan revealed increased ventricular size. On exploration of the shunt, manometry with saline confirmed blockage of the catheter distal to the valve. On re-opening the abdominal wound, the peritoneal catheter was found to be knotted, 2 cm from the end. This segment of the catheter was replaced, with resolution of symptoms, post-operatively. The present case illustrates that a knot in the peritoneal catheter is an extremely rare cause of shunt malfunction. Possible mechanisms underlying it are discussed.