Several lines of evidence support for the role of angiotensin-converting enzyme (ACE) in Alzheimer's disease (AD) patients. Most human genetic studies have focussed on ACE insertion (I)/deletion (D) polymorphism and have yielded conflicting results. We have evaluated the association of ACE polymorphism with serum ACE activity in 95 AD patients and 110 healthy controls from north Indian population. In Alzheimer's patients a higher frequency of D allele was detected (I/D ratio 0.53:0.47) compared with the control group (I/D ratio 0.54:0.45), the difference being not statistically significant (p > .05). AD patients were found to be more homozygous for the D allele (26.3%) compared with controls (20.8%). The observed genotype distribution was in agreement with Hardy-Weinberg equilibrium. We observed that the D/D genotype is more in patients with a higher serum ACE activity. The D allele and the D/D genotype in AD patients may influence increased risk of cognitive impairment.