The renin-angiotensin system plays an important role in the maintenance of blood pressure homeostasis. The angiotensin-converting enzyme (ACE) converts angiotensin I into angiotensin II. Angiotensin II, which binds the angiotensin II type-1 receptor (AT1R), is a potent vasoconstrictor. On a pathophysiological basis, both ACE I/D and AT1R A1166C polymorphism lead to an enhanced activity of the angiotensin II-AT1R axis, thereby possibly contributing to circulatory disturbances. A mutually facilitatory effect may be presumed between the two polymorphisms. We examined whether this synergistic effect is involved in the evolution of different types of ischemic stroke. Genetic and clinical data on 308 consecutive patients with acutely developing ischemic stroke were analyzed. Atotal of 272 stroke and neuroimaging alteration-free subjects served as a control group. Univariate and logistic regression statistical approaches were used. The ACE D allele combined with the AT1R 1166C allele did not yield a risk of ischemic stroke. However, the co-occurrence of the homozygous ACE D/D and at least one AT1R 1166C allele was more frequent in the ischemic stroke group than in the control group (22.4 vs 11%, p < 0.005, OR, 2.33; 95% CI, 1.46-3.7). After specific subgroup analysis, this synergistic association was even stronger for small-vessel ischemic stroke (OR, 3.44; 95% CI, 1.9-6.24; p < 0.0005). Multivariate logistic regression analysis of the data confirmed this association (adjusted OR, 3.54, 95% CI, 1.88-7.16; p < 0.0005). Our results demonstrate that ACE D/D and AT1R 1166C polymorphism were associated with the development of small-vessel ischemic stroke through a mutually facilitatory interplay between them. Genetic interactions might contribute to the altered functional network in renin-angiotensin system in vascular disorders.