To describe the clinical features, imaging findings and pathological aspects of breast cancer diagnosed in women under the age of 40 years. A retrospective, descriptive study was performed through analysis of medical records between November 2008 and August 2012. One hundred and twenty (120) patients were included, of whom 112 underwent mammography, 113 underwent ultrasonography, and 105 underwent magnetic resonance imaging (MRI). The histopathological data was obtained in most cases from post-surgical analysis, which was available for 113 patients. The mean age at diagnosis of primary breast cancer was 34 years. Only 11 patients (9.0%) had a family history of breast or ovarian cancer in first-degree relative. Ninety-two (92) patients sought medical attention after showing breast symptoms, and the presence of a palpable nodule was the main complaint. One hundred and twenty-two (122) primary tumors were diagnosed, of which 112 were invasive (95%). The most common histological type was invasive ductal carcinoma (73.8%). Luminal B was the predominant molecular subtype (42.6%). Ultrasonography was positive in 94.5% of the cases and the most common finding were nodules (94.8%). At mammography, the malignancy was observed in 92.8% and the presence of suggestive calcifications was the dominant feature. The MRI was positive in 98% of patients, and mass lesions were the most common. Most cases of breast cancer diagnosed in patients under the age of 40 years, in our population, had symptoms at diagnosis and tumor with more aggressive biological behavior. Despite the ultrasound has been the most widely used method, we found improved characterization of breast lesions when also used mammography and MRI.