Background: To evaluate ultrasound (US) characteristics and BI-RADS (Breast imaging-reporting and data system) of malignant breast masses in women <40 years and to compare with older patients. Methods: In a retrospective, descriptive-analytical study, we assessed the US images and BI-RADS category of 78 malignant masses with a final pathology of invasive ductal carcinoma (IDC, NOS type). Results: Overall, the most frequent US descriptors of IDC were indistinct margin (45%), irregular shaped (63.5%), posterior shadowing (38.8%), heterogeneous internal echogenicity (56.3%) and non-parallel orientation (76.3%). In this study, most malignant masses of young patients were categorized as BI-RADS 4a while in the older patients (over 40), they were mostly BI-RADS 4b and 5 with P = 0.03 and odds ratio (OR) of 2.57 (95% confidence interval (CI), 0.74-8.8). In addition, the mean dimension of the mass in young cases was greater (18.3 mm) compared with older patients (13.2 mm) with P value of 0.04 and OR of 3.8 (95% CI, 1.1-13.4). Conclusion:Similar to previous studies, malignant masses were diagnosed in greater dimensions in younger cases which may be due to the delay in diagnosis, the rapid growth of the tumor and the absence of routine screening guidelines. Radiologists should be aware of the possibility of malignancy in palpable slightly suspicious masses (BI-RADS 4A) in young cases.