Objective: This retrospective research aimed to evaluate the results of treatment outcome and clinicopathological features of breast cancer patients under 40 years old. Material and methods: A total of 80 patients who were receiving radiotherapy and/or chemotherapy treatment for breast cancer (<40 years old) were included in the study. Results: First-degree relatives with breast cancer history (p=0.028), oestrogen receptor positivity (p=0.012) and progesterone receptor positivity (p=0.017) were associated with overall survival. No prognostic factors were found in the multivariate Cox regression analysis for overall survival. In multivariate Cox regression analysis, we found breast-conserving surgery type (hazard ratio = 6.104. 95% confidence interval = 1.037–53.928, p=0.045), lymphovascular invasion presence (hazard ratio = 0.127, 95% confidence interval = 0.016–1.025, p=0.005) and curative radiotherapy doses (hazard ratio = 185.976, 95% confidence interval = 5.342–6474.1, p=0.004) as independent prognostic factors for disease-free survival. Overall, survival of 1, 3 and 5 years was 88%, 74% and 65%, respectively. Median was 48±2.6 (42.8–53.19) months. Also, 1-, 3- and 5-year disease-free survival was 85%, 67% and 27%, respectively. Median was 30±1.8 (27.4–32.5) months. Conclusion: Breast cancer patients under the age of 40 years are highly heterogeneous and are a complex patient group. The prognosis is worse in these patients, and prognostic factors and pathological subtypes should be taken into consideration when making treatment decisions.