Self-efficacy has a positive effect on health behaviors, symptom control, compliance with cancer treatment, and quality of life. This study aims to describe the quality of life and self-efficacy of Turkish breast cancer patients undergoing chemotherapy. The sample consisted of 141 patients. Data was gathered using a Patient Information Form, the Functional Assessment of Cancer Therapy-Breast Cancer (FACT-B), a scale about Strategies Used by Patients to Promote Health and the Rotterdam Symptom Checklist. All quality of life dimensions were negatively affected at a significant level. Following commencement of chemotherapy, there was an increase in the negative effect on physical well-being, emotional well-being and additional concerns subscales and total FACT-B and their self-efficacy was negatively affected to a moderate degree. However, a significant degree of change did not occur in the self-efficacy. During treatment the physical symptoms and psychological distress increased and the activity level was negatively affected. The quality of life and self-efficacy were influenced by personal and medical characteristics, showing consistency with similar studies. Because there are negative effects of cancer and chemotherapy on patients' quality of life and self-efficacy, nurses need to focus on designing psychosocial interventions to improve their self-efficacy and quality of life.