BACKGROUND: Evaluation of the cosmetic outcome after breast-conserving surgery (BCS) differs depending on the evaluator. We performed a clinical trial to examine the differences between assessments of cosmetic outcomes performed by a surgeon, patients, and a nurse as a third party after BCS; the evaluation was performed two times (at 3 months and 9 months after surgery). Similarly, we identified factors most significantly affecting the overall cosmetic outcomes. METHODS: Sixty-eight patients with primary breast cancer who had undergone BCS between September 2017 and December 2018 were consecutively enrolled in the study. Breast shape, symmetry, hardness, scarring, and overall outcomes were evaluated by a surgeon, patients, and a nurse via a questionnaire. RESULTS: Intraclass correlation coefficients (ICCs) for the 3- to 9-month comparisons of the surgeon, patients, and nurse were 0.73, 0.64, and 0.29, respectively. The ICCs for the surgeon-patient, nurse-patient, and surgeon-nurse comparisons (3 months/9 months) were 0.49/0.44, 0.34/0.10, and 0.41/0.51, respectively. The partial regression coefficient for shape was 0.45 (p = 0.003)/0.61 (p = 0.001), 0.37 (p = 0.005)/0.50 (p < 0.001), and -0.08 (p = 0.48)/0.58 (p < 0.001) for evaluations performed by the surgeon, patients, and nurse, 3 months and 9 months, respectively. CONCLUSION: With reproducibility, only moderate agreement was observed between the surgeon and the patients. Breast shape was identified as the most important factor affecting cosmetic outcomes.