Abstract An analysis of the results of advanced (A) and ordinary (O) level examinations revealed a lower pass-rate, lower distinction-rate, and lower average mark when the examinations were taken during the premenstruum or during menstruation. This was most striking in girls whose menstrual loss continued longer than 6 days and those with menstrual cycles exceeding 31 days. In 42% of the ninety-one girls, whose normal menstrual pattern was known, the stress of the O-level examinations produced an alteration in their menstrual cycle. This resulted in more girls menstruating during examination week than would have been expected from their normal pattern. The tendency was for the cycle to be lengthened rather than shortened, but some girls had temporary amenorrhœa during the examination month.