The hormone levels of 13 polymenorrhoeic adolescent girls were evaluated. Twenty-eight adolescent girls with regular menstruation served as controls. Serum concentrations of luteinizing hormone, follicle-stimulating hormone, prolactin, oestradiol, progesterone, testosterone, androstenedione, dehydroepiandrosterone, dehydroepiandrosterone-sulphate, and sex hormone binding-globulin were determined in blood samples obtained over one menstrual cycle. In girls with polymenorrhoea the frequency of ovulation was significantly lower than in the control group as estimated on the basis of the progesterone level five days before the onset of menstruation. Low progesterone and oestradiol levels during the late part of the cycle were the main abnormalities of polymenorrhoeic adolescent girls. No differences in the other hormonal parameters were found. The duration of menstrual bleeding was similar in both groups.