Background This prospective, double-blind and randomized study is designed to determine the effect of menstrual cycle on the injection pain of propofol.,Methods Seventy-two patients scheduled for elective surgery under general anaesthesia were divided into two groups according to the phase of the menstrual cycle. Patients were at follicular phase (Pd 8-12) in Group F (n = 36) and luteal phase (Pd 20-24) in Group L (n 36). Injection pain was evaluated with 10-point numeric rating scale after 25% of the total propofol dose was injected over 20 s.,Results There were no significant differences in terms of patient characteristics (P>0.05). The mean propofol pain score was found 1.81 +/- 2.30 in Group F and 4.83 +/- 3.09 in Group L. Group L was found to have higher propofol injection pain scores than Group F (P<0.001).,Conclusion We conclude that the menstrual phase changes the perception of pain due to propofol injection, which is higher in the lutheal phase. In clinical practice, the phases of the menstrual cycle may have a significant role on injection pain of propofol in woman. Eur J Anaesthesiol 2010;27:425-427