Background/aim: To evaluate the diagnostic value of vascular indices obtained using 3D power Doppler ultrasound (3DPDUS) in differentiation of benign and malignant thyroid nodules. Materials and methods: Sixty-seven patients (56 female, 11 male, mean age 44.6) with 81 thyroid nodules exhibiting mixed (peripheral and central) vascularization patterns and with the largest diameter of between 10 mm and 30 mm were prospectively evaluated using 3DPDUS. Nodule volume, vascularization index (VI), flow index (FI) and vascularization flow index (VFI) were calculated using Virtual Organ Computer-aided Analysis (VOCAL) software, and these indices were then compared with regard to the cyto-histopathology-based diagnosis. The optimum cutoff values for the differentiation of benign and malignant nodules were identified, and diagnostic efficacy was calculated using receiver operating characteristic (ROC) analysis. Results: Fifty-six of the 81 nodules included in this study were diagnosed as benign and 25 as malignant. Vascular indices in malignant nodules were significantly higher than those in benign nodules (p<0.05). In benign nodules, mean VI was 11.61 ± 6.88, mean FI was 39.75 ± 3.93 and VFI was 4.82 ± 2.94, compared to 18.64 ± 12.81, 41.82 ± 4.43 and 8.17 ± 6.37, respectively, in malignant nodules. The area under the curves (AUCs) were calculated as 0.68, 0.61 and 0.67 for VI, FI and VFI, respectively. At optimal cut-off values of 10.2 for VI, 40.8 for FI and 5.5 for VFI, sensitivity and specificity were 72%/55.4%, 68%/57.1% and 68%/67.9%, respectively. Conclusion: 3DPDUS could be useful in the characterization of thyroid nodules. Key words: Thyroid nodules; three-dimensional power Doppler ultrasound; vascular indices; VOCAL