Objective: To investigate the impact of body composition on quality of life (QoL) and side effects in patients with early-stage breast cancer during anthracycline-based chemotherapy. Setting: Prospective research and follow-up of patients at the Department of Medical Oncology, University Hospital for Tumors, UHC Sestre milosrdnice, University of Zagreb from December 2018 to April 2020. Subjects and methods: The study population consisted of premenopausal patients treated for early-stage breast cancer using anthracycline doxorubicin-based neoadjuvant / adjuvant chemotherapy (AC protocol). During each of the four planned visits to the chemotherapy application, the patients completed the standardized questionnaires QLQ-C30 and QLQ-B23 and from the 2nd to the 4th cycle a questionnaire on side effects experienced in the period from the previously applied chemotherapy cycle to the next cycle. The physician then completed side effect questionnaire by assessing the severity of the side effects. Anthropometric measurement and weighing were then performed on a bioimpedance measuring device. Results: The study included a total of 68 patients, mean age 52.6 years. Analysis of body composition indicated a slight decrease in body weight, with no statistically significant changes. Quality of life analysis indicated a marked decline in some sub- domains during therapy, most pronounced for nausea and vomiting, shortness of breath and fatigue; these changes were most pronounced between the first and fourth cycle of treatment. Analysis of the BR23 questionnaire indicated a slightly more pronounced reduction in sub-domains during the treatment cycle, with the largest change observed for hair loss and systemic therapy. Body composition was associated with physical and sexual functioning and in subsequent treatment cycles to increased fatigue. Conclusion: The study showed that there were no significant changes in body weight and body composition of patients with early-stage breast cancer during treatment with anthracycline-based chemotherapy, and since no significant change in body composition was observed during treatment, no correlation was observed between patients' body composition and the extent of reported adverse reactions. Quality of life is impaired during chemotherapy treatment, and body composition can significantly affect individual QoL sub-domains.