Fourteen patients with ventricular cerebrospinal fluid shunts in place for chronic hydrocephalus presented with a history and neurological deficits usually associated with high intracranial pressure (ICP) caused by an obstructed shunt system. However, the symptoms were characteristically present when the patient was upright and active, and were usually relieved by lying down. The symptoms of intermittent headache, nausea, emesis, lethargy, and diplopia were associated with paresis of upward gaze or minimal strabismus. Measurement of ICP showed unexpected dramatically low levels with a marked drop in pressure when the patient was in the upright position, whereas ICP was near normal when the patient was supine. The low ICP was corrected by insertion of a high-pressure Flo-Control valve into the shunt system already in place. Postoperatively, the immediate clinical improvement and more normal ICP measurements were striking. The important clinical finding in this group of patients was the presence of disabling symptoms which occurred when the patients were up and active and which were relieved by lying down. Measurements of ICP with the patient in the supine and then in the upright position were critical in establishing an accurate diagnosis of symptomatic low ICP in these hydrocephalic patients with indwelling shunts. With the patient in the Trendelenburg position, ICP showed a marked increase, as expected; in some patients this position was prescribed as treatment for several days before surgery.