Studies carried out on sheep's have suggested possible effects of hematological parameters on coronary flow reserve (CFR). However, there is no study published investigating possible effects of hematological parameters and blood viscosity on coronary haemorheology and CFR in humans. We investigated the possible effects of hematological parameters on CFR using transthoracic echocardiography. Methods: This study was conducted on 142 healthy subjects free of known cardiovascular risk factors between ages of 18-50 years. Hematological parameters were measured using an automated blood analyzer. Transthoracic echocardiographic examination including CFR measurement was performed on each subject using an Acuson Sequoia C256 (R) Echocardiography System. Results: The study group (142 subject) was divided into two according to the mean CFR values as the group with lower than mean CFR value and the group with upper than mean CFR value. Hematocrit (42.21 +/- 3.86 vs 40.27 +/- 3.97, P=0.004), LDL cholesterol, hsCRP values differed between the lower CFR and higher CFR groups. The other hematological parameters including hemoglobin value were similar between the two groups. hsCRP (beta=-0.334, P=0.001), mitral E/A ratio (beta=0.119, P=0.024), and haematocrit (beta=-0.161, P=0.064) values were the independent predictors of CFR. In women's subgroup, age, hemoglobin, hematocrit, and hsCRP values tended to differ between the lower CFR and the higher CFR subgroups. In men's subgroup systolic blood pressure, hsCRP, and mitral E velocity were different between the two groups. Conclusion: In addition to the coronary endothelial and coronary microvascular functions, hematocrit and blood viscosity might have some effect on coronary haemorheology and CFR.