Aim: Monitoring the nutritional status of cancer patients is crucial. We aimed to assess the associations between nutritional status, oxidative parameters and quality of life before, during, and after chemotherapy (CT) in breast cancer patients. Material and Method: Clinical, anthropometric, demographic, quality of life, laboratory (i.e., blood oxidative marker level) and dietary intake data of breast cancer patients who planned to have two cycles of CT were recorded. All collected data were compared between pre-CT, mid-CT, and post-CT time points. Results: Fifty women who were diagnosed with breast cancer and would start CT treatment were included in the study. Post-CT body weight and BMI was significantly lower than the pre-CT values (p˂0.001). They both turned back to pre-CT levels at the end of CT with a decrease in total daily caloric and macronutrient intake. Consumption of specific food groups such as milk or yogurt, cheese, eggs, and sugar significantly decreased compared to their pre-CT consumption levels (p˂0.001, p˂0.001, p=0.017, and p=0,01). Blood oxidative stress marker analysis revealed a significant reduction in GPx levels with CT (p=0.007). Analysis of the quality-of-life scores revealed that post-CT scores were significantly higher than the pre-CT scores while the life quality was lowest in the mid-CT period. Conclusion: Breast cancer patients have a lower dietary intake during CT, which is associated with a lower intake of specific food groups. Adequate caloric intake and antioxidant intake should be recommended to these patients during CT via dietary counseling to maintain healthy anthropometric measures and oxidative hemostasis.