The effects of the timing of surgery, fine needle aspiration cytology (FNAC) and mammography within the menstrual cycle were investigated retrospectively in 211 premenopausal breast cancer patients. The day of surgery within the menstrual cycle was known for 157 women whose menstrual cycles were regular. Recurrence and survival were closely similar whether surgery was performed during days 3-12 or at other times (days 0-2 or 13-32) in the cycle. Outcome was also unrelated to the timing of mammography which was known in 101 cases. There were no significant differences in recurrence or survival associated with the timing of FNAC when all patients were considered. In the subset of lymph node negative patients (n = 32), however, FNAC outside days 3-12 was associated with a significantly decreased rate of relapse (RR = 0.25, 95% CI = 0.07-0.9) and death (RR = 0.10, 95% CI = 0.02-0.9). These patients, however, also had significantly smaller tumours (median = 2 cm, interquartile range = 2-3 cm) than patients having FNAC on days 3-12 of the cycle (median = 3, interquartile range = 2-4) (Mann-Whitney: z = 2.11, P = 0.04). We have not confirmed that surgery or FNAC during days 3-12 of the cycle is associated with a poorer outcome than interventions during other parts of the cycle.