Gender is an important risk factor for postoperative nausea and vomiting. Menstrual cycle phase has also been implicated as a further variable in female patients. It is not clear whether a history of an irregular menstrual cycle is a significant variable and studies to date have not examined this. The influence of an irregular menstrual cycle on nausea and vomiting after laparoscopy was evaluated in a blinded, prospective, observational study. One hundred and fifty-nine patients scheduled for laparoscopy and dye investigation were included. Anaesthetic technique and postoperative analgesia were standardised. In comparison with patients who had a regular cycle, an irregular menstrual cycle increased the incidence of nausea or vomiting from 20.5% to 40.5% (absolute difference 20%; 95% CI 0.03-0.37, p = 0.01). Other variables were similar between the groups. An irregular menstrual cycle appears to increase the risk of postoperative nausea and vomiting.