4112 women completed a retrospective questionnaire indicating when during their last menstrual cycle they felt their well-being and sexual interest to be at their best and worst. The commonest pattern was for well-being to be lowest during the premenstrual and highest during the postmenstrual week. Sexual interest was strongly associated with well-being, suggesting that variations of well-being have a powerful effect on sexuality in the majority of women. Oral contraceptive users, though broadly similar in their reported pattern, were less likely to show peaks and troughs of well-being and highs and lows of sexual interest. This was most evident in the subgroup of monophasic pill users who showed the least tendency to variations in both well-being and sexual interest, and a greater tendency to show either peaks or troughs of well-being during menstruation. Triphasic pill users were intermediate between monophasic and non-pill users.