Idiopathic normal pressure hydrocephalus (INPH) is a potentially treatable form of dementia but its diagnosis is difficult and the effectiveness of shunting remains controversial. This study investigates the clinical outcomes of ventriculo-peritoneal shunting in a controlled trial of 33 consecutive patients with INPH. Mean age was 77.2 years (range 58-92 years) and the duration of symptoms was 4.6 years (3 months-14 years). Nineteen patients underwent shunt surgery. At 3-4 months follow-up, patients who had undergone shunt surgery, compared to those who had not (controls), had significantly better global change ratings (median Clinician's Interview Based Impression of Change with Carer Input rating of 2 [moderately improved] versus 6 [moderately worsened], respectively, p<0.001), had increased Mini Mental State Examination scores by 5 points (p<0.001) and were 6.3s faster on the Timed "up and go" test (p=0.008). We conclude that ventriculo-peritoneal shunting is associated with improved clinical outcomes for patients with INPH.