Real-time sono-elastography is an ultrasound-based technique used to estimate tissue elasticity. Several publications have reported that this method has the ability to differentiate between malignant and benign breast lesions. However, on the basis of current literature, sono-elastography returned false-negative results in 25% of cases with certain lesions, such as mucinous carcinoma. Our data indicate that elastography has higher specificity (96.5% vs. 84.4%) and lower sensitivity (86.9% vs. 93.9%) than B-mode ultrasound. Our evidence suggests that elastography performs significantly worse in lesions ≥20 mm in diameter (sensitivity = 61.1%, specificity = 97.2%) than in lesions <20 mm in diameter (sensitivity = 92.6%, specificity = 96.2%). Furthermore, elastography returned false-negative results in all cases mucinous carcinoma. Finally, in eight cases we obtained a valid elastogram. Our data indicate that this finding is probably due to tumor depth.