Background: Worldwide, breast cancer is the leading type of malignancy in women. For premenopausal women, the disease brings much higher risk as it is usually more aggressive with worse prognosis. Patients and Methods: In this retrospective study, 92 women treated at the Department of Oncology and Radiotherapy in Pilsen were selected from a basic cohort of 356 women under 35 years of age with breast cancer who were diagnosed between 2006 and 2015. The control group comprised 100 postmenopausal women over 65 years of age who were treated for invasive breast cancer. Results: Overexpression of HER2/neu protein and a triple-negative immunoprofile and basal-like phenotype of cancer were more frequently seen in the women under 35 years of age. In addition, malignant cells were poorly differentiated and more aggressive, and prognostically favourable types were not often seen, in these women. In terms of the course of disease, the outcome was worse for the younger patients, and complete remission was reached less frequently and more cases of advanced disease and death due to the malignancy were detected. Conclusion: The incidence of invasive breast cancer in young women is low, representing around 2% of all cases of the disease, but this group of patients is prognostically very important. The cancers at such a young age are usually more aggressive (higher mitotic activity and higher grade), and prognostically worse types, such as triple-negative or basal-like, are seen significantly more often in younger patients. This retrospective study confirmed this premise. Moreover, breast cancer in young women is more often associated with genetic predisposition (e. g., hereditary mutations in BRCA1 and BRCA2 genes) than in older women.