Purpose: To prospectively compare the diagnostic performance of radiologists by using conventional ultrasonography (US) and tissue harmonic imaging for the differentiation of benign from malignant solid breast masses, with histologic results used as the reference standard. Materials and Methods: The study was approved by the institutional review board, and informed consent was obtained from all patients. Images were obtained with, conventional US and tissue harmonic imaging in 88 patients (age range, 25-67 years; mean age, 45 years), with 91 solid breast masses (30 cancers and 61 benign lesions) before excisional or needle biopsy. Three experienced radiologists, who did not perform the examinations, independently analyzed the US findings and provided a level of suspicion to indicate the probability of malignancy. Results were evaluated by using κ statistics and receiver operating characteristic (ROC) analyses. Results: Regarding the descriptions of US findings, echogenicity (κ = 0.205) was the most discordant between conventional US and tissue harmonic imaging, followed by margin (κ = 0.495), lesion boundary (κ = 0.495), calcifications (κ = 0.537), posterior acoustic transmission (κ = 0.546), echotexture (κ = 0.586), shape (κ = 0.591), and orientation (κ = 0.594). The area under the ROC curve (Az) for conventional US and tissue harmonic imaging was 0.84 and 0.79, respectively, for reader 1; 0.88 and 0.85, respectively, for reader 2; and 0.91 and 0.89, respectively, for reader 3. The overall Az value for the three readers was 0.88 for conventional US and 0.84 for tissue harmonic imaging (95% confidence interval: -0.0950, 0.1646; P = .595). Conclusion: The performance of the radiologists with respect to the characterization of solid breast masses as benign or malignant was not significantly improved with tissue harmonic imaging. © RSNA, 2006.