Infection and obstruction remain significant problems associated with shunts. We studied the clinical and pathological findings in 29 patients who underwent surgical procedures to revise malfunctioning ventriculoperitoneal shunts. The preoperative diagnosis for shunt malfunction was infection in 8 cases, and obstruction in 18 cases. A pathologist NK independently examined the shunt hardware removed from the patients without knowledge of the clinical diagnosis. There were 19 ventricular catheters, 15 valves, and 14 abdominal catheters examined. Fourteen of the patients had had three or more shunt revisions. Three of these patients had evidence of some hypersensitivity reaction due to the shunt hardware (multiple eosinophils and giant cells in tissue enveloping shunt hardware). Of the 15 patients who had two or less shunt revisions, none had evidence of hypersensitivity on pathological examination of the shunt hardware. We conclude that pathological examination of shunt hardware is helpful in understanding noninfectious causes of shunt malfunction. We have identified a subgroup of patients who developed a hypersensitivity-like reaction around the shunt that in certain instances may lead to shunt malfunction. It is important to identify these patients for they may be more likely to have multiple episodes of shunt malfunction.