BACKGROUND In 1989 it was suggested that the timing of surgery during follicular or corpus luteum phase of the menstrual cycle could substantially influence the survival time in premenopausal breast-cancer patients [1]. Several researchers analysed their data, but in general they could not confirm this observation. Nevertheless, two other reports in that series found some influence of the time of surgery during menstrual cycle on prognosis, although, they suggested different time periods as favourable [8, 9]. OBJECTIVE The purpose of this study was to investigate the effect of timing of surgery in a study population which was much larger than the previous studies. METHODS All premenopausal patients with primary breast cancer who had been operated at one clinic between 1st of January 1980 and 31st of December 1990 were considered eligible for the study. Based on the day of surgery during the menstrual cycle all patients were grouped according to three proposed categories and to an additional one which consisted of the favourable days from the three proposed categories. We investigated the effects of several prognostic factors and of the timing of surgery on three endpoints: recurrence free survival, distant disease free survival, and overall survival. RESULTS The traditional prognostic factors had effects in agreement with the literature. Of the different categories for timing of surgery none had a significant influence; the strongest in univariate analysis had a p-value of 0.10 and the trend was in the opposite direction to the one proposed in the literature. These results did not change in multivariate analysis with the Cox model. CONCLUSIONS Timing of surgery does not have an important effect on the prognosis of breast cancer patients. Incorporating other results from the literature, we conclude that it has at most a weak effect. CONCLUSIONS At present there is no need to incorporate the day of menstrual cycle in the planning of surgery for premenopausal breast cancer patients.