Introduction/Objective. Circle of Willis (CoW) provides the most significant collateral flow in the presence of significant stenosis or occlusion of internal carotid artery. In terms of collateral flow ?incomplete? type and ?complete? type of CoW can be recognized. Patients with carotid artery disease with incomplete CoW have lower cerebrovascular reactivity and higher risk for stroke. Cerebrovascular reactivity refers to the residual capacity of dilatation of cerebral blood vessels in the condition of insufficient blood flow. In this study we analyzed changes in cerebrovascular reactivity after carotid endarterectomy in asymptomatic patients with respect to complete and incomplete CoW morphology. Methods. In this study in 97 patients with asymptomatic carotid artery disease we measured cerebrovascular reactivity before and after carotid endarterectomy by using method of ?apnea test? and ?breathholding index? (BHI). Patients were divided into two following groups: patients with ?complete? CoW and ?incomplete? CoW based on non-contrast magnetic resonance angio performed previously to the operation. Descriptive statistics, univariate analysis, and ANOVA for comparison of BHI values between groups were used. Results. The results showed significant increase in cerebrovascular reactivity at the side of stenosis in both groups of patients with complete CoW (BHI value increased from 0.897 to 1.090; F (1.65) = 30.788, p < 0.0005, parc. ?2 = 0.321) and incomplete CoW (BHI value increased from 0.690 to 1.010; F (1.27) = 62.318, p < 0.0005, parc. ?2 = 0.698) and the more significant increase in the group of incomplete CoW compared to the group with complete CoW (F (1.92) = 4.557, p = 0.035, parc. ?2 = 0.047) Conclusion. In most asymptomatic patients, cerebrovascular reactivity restores to normal following carotid endarterectomy. Parameters of cerebrovascular reactivity are lower in patients with incomplete CoW and the increase after carotid endarterectomy is more significant in such patients.