The purpose of our study was to analyze the accuracy of quantitative analysis of tumor vascularity on power Doppler sonograms in differentiating malignant and benign solid thyroid nodules using tumor histologic evaluation as the reference standard. Methods. Eighty-six solid thyroid tumors (46 malignant and 40 benign) in 56 consecutive patients (mean age +/- SD, 53.1 +/- 11.6 years; 12 male and 44 female) referred for the surgical treatment were included in our study. Visual and qualitative analysis of patterns of nodule vascularity was performed for all tumors. Quantification of the power Doppler sonograms was performed with normalized and weighted vascular indices (VIs). The accuracy of sonographic criteria for thyroid cancer was evaluated with univariate analysis. Results. Among benign thyroid tumors, there was a statistically significant increase in the levels of intranodular vascularization with an increase in tumor size (P < .001). In all tumors, increased intranodular vascularization showed 65.2% sensitivity, 52.5% specificity, and 58.9% overall accuracy in differentiation between benign and malignant thyroid lesions. In tumors smaller than 2 cm, it had 65.5% sensitivity, 85.7% specificity, and 72.1% overall accuracy. Quantitative analysis of tumor vascularity significantly overperformed visual analysis of power Doppler patterns (P < .05). Among thyroid lesions with diameters of less than 2 cm, a normalized VI of greater than 0.14 had 72.4% sensitivity, 100% specificity, and 86.2% overall accuracy. A weighted VI of greater than 0.24 showed compatible results, with 69.0% sensitivity, 100% specificity, and 84.5% overall accuracy. Our study indicates that in small thyroid nodules, quantitative analysis of tumor vascularity has benefits over visual inspection and can be useful in differentiation between benign and malignant thyroid tumors.