The potential influence of lunar phases on human life has been widely discussed by the lay press. The purpose of this study was to find out whether the timing of surgery during particular lunar phases influences the survival of breast cancer patients. It has been postulated that breast cancer surgery performed during the waxing moon, or particularly at full moon, is associated with a poorer outcome. We tested this hypothesis by evaluating the overall survival for 3,757 consecutive patients with invasive breast cancer. All patients underwent either modified radical mastectomy or breast conserving surgery plus radiotherapy, followed by adjuvant cytotoxic or hormonal therapy. The date of definitive surgery was allocated to the lunar phases. 1,904 (50.7%) patients were operated on during the waxing moon and 1,853 (47.3%) during the waning moon. The median follow-up was 74 months (range 1-372 months). The mean age at primary surgery did not differ significantly in the two groups 58.39 (SD 13.14) versus 58.34 (12.75) (p >0.05, t-test). Breast cancer stages at initial diagnosis were evenly distributed according to the lunar phases (p = 0.325; chi-square). Survival curves were plotted according to the method of Kaplan-Meier. No significant differences were observed when timing of surgery was allocated to the lunar phases (p = 0.4841, log-rank). Subgroup analysis of premenopausal patients revealed similar results (p = 0.2950, log-rank; n = 1072). Using multivariate Cox modelling, we found a significant association between the patient's age, stage of disease and survival, whereas no association with survival was observed for the timing of surgery (RR= 1.062; 95% CI, 0.970-1.163; p = 0.1937). No significant differences in overall survival of breast cancer patients were observed when timing of breast cancer surgery during the lunar cycle was considered. Although this was not a prospective randomized trial, the statistical magnitude of the results do not support any recommendations for scheduling patients for surgery at any particular day of the lunar phase.