BACKGROUND It is unclear whether isolated coronary artery ectasia (iCAE) is associated with whole blood viscosity (WBV). In the present study, we investigated WBV in coronary artery ectasia (CAE) patients. METHODS Seventy-eight patients with iCAE and 83 controls with normal coronary arteries were selected from 12290 patients who underwent coronary angiography between January 2014 and December 2017. WBV was calculated with a validated equation from hematocrit and total plasma protein levels for a low (LSR) and high (HSR) shear rate. RESULTS Baseline demographic characteristics and medical history of the groups were similar. The mean level of C-reactive protein (2.1 ± 0.53 vs. 1.93 ± 0.44; p = 0.042) and total protein (7.2 ± 0.3 vs. 7.0 ± 0.6; p = 0.009) were significantly higher in the iCAE group than in the control subjects. Both HSR (4.57 ± 0.6 vs. 3.9 ± 0.7; p < 0.001) and LSR (33.5 ± 9.6 vs. 25.1 ± 9.2; p < 0.001) levels were significantly higher in the iCAE group than in the control group. In ROC analysis, a cut-off value of 4.19 WBV for HSR had an 80.8% sensitivity and 72.3% specificity [area under the curve (AUC): 0.779, 95% CI 70.6-85.1; p < 0.001] and a cut-off value of 27.5 WBV for LSR had an 80.1% sensitivity and 72.3% specificity for predicting iCAE (AUC: 0.788, 95% CI 71.4-86.2; p < 0.001). In multivariate analysis, both LSR (p < 0.001, OR 1.10, 95% CI 1.05-1.15) and HSR (p < 0.001, OR 4.60, 95% CI 2.33-9.09) were independent predictors for the presence of iCAE. CONCLUSIONS In the present study, we determined that in WBV, both HSR and LSR were significantly higher in the iCAE group than in the control subjects, and that this may be a possible cause of iCAE.