Objective: To identify the relation between cerebral collaterals (according to TCD based grading system) and cerebrovascular reserve (measured by SPECT) in patients with symptomatic chronic total carotid occlusion. In addition, to study the effect of vascular risk factors on both collaterals and cerebrovascular reserve. Methods: Thirty-four patients with chronic total carotid occlusion diagnosed by ultrasound and had ischemic symptoms either stroke or TIA. Each was subjected to : 1- Clinical assessment. 2- Cerebrovascular reserve assessment with SPECT with dipyridamole stress. 3- Grading of cerebral collaterals using transcranial duplex and Doppler (TCD).then statistical analysis was applied using univariate and multivariate analysis methods. Results: CVR showed significant positive correlation to collaterals grading with P value < 0.001 and Spearman correlation coefficient 0.686. Hypertension and diabetes were predictors for poor collaterals with (p value 0.049 and 0.045, odds ratio 11.5 and 0.131 with 95% confidence interval 1.01 to 131.16 and 0.018 to 0.953 respectively). Smoking was significantly related to both poor cerebrovascular reserve capacity and poor collaterals with P value 0.01. Smoking also was related to poor collaterals with P value 0.03 but CI (-2.14 to -.014). Conclusion: cerebral collaterals has important role in the maintenance of cerebrovascular reserve in patients with total carotid occlusion. Furthermore, the proposed TCD-based collateral grading system showed good reliability indices when validated against some of the SPECT finding.