Previous reports have suggested an association between the insertion/deletion (I/D) polymorphism of the angiotensin-converting enzyme (ACE), cardiovascular disease, and cognitive performance. Normal pressure hydrocephalus (NPH) is considered to be an example of reversible dementia although the clinical improvement after shunting varies from subject to subject. An association has been suggested between vascular risk factors and the development of NPH. The ACE plays a major role in vascular pathology and physiology. In the present study we investigated the distribution of an ACE gene insertion/deletion polymorphism in 112 patients diagnosed with NPH and in 124 controls. We also evaluated the role of this genetic polymorphism in cognitive functioning before and following surgery in a subgroup of 72 patients. No differences in genetic or allele distributions were found between patients and healthy subjects, but among patients, carriers of D/D or D/I genotypes obtained less cognitive benefit following shunt surgery, especially on measures of memory and frontal function. Our data support previous findings in other conditions indicating that possession of at least one D allele is associated with poorer cognitive performance.