A simple and reliable bedside infusion test is described to evaluate shunt-system patency in adult patients with treated "normal" pressure hydrocephalus. Fifteen consecutive patients with ventriculoperitoneal shunts were tested by a two-part infusion test between 2 days and 32 months after shunt surgery. When we used the criteria presented, shunt patency could be established with certainty, and shunt malfunction could be established with relative certainty. Shunt malfunction was verified surgically.