To establish whether there is a variation in the incidence of onset of acute anterior uveitis (AAU) in women during the phases of the menstrual cycle.Prospective open study in women attending the Acute Referral Centre with a first or recurrent attack of AAU.There was a significant increase in the incidence of AAU during the late stages of the menstrual cycle.The onset of AAU is partially dependent on the levels of either oestrogen or progesterone, or both. The withdrawal of the proven anti-inflammatory effects of these hormones may provoke the onset of uveitis.