Patients with severe idiopathic constipation are almost exclusively women of reproductive age. To investigate the possibility of a sex hormone abnormality in this condition, we have compared a range of sex hormones during the follicular and luteal phases of the menstrual cycle in 23 healthy women (mean age 33 years) with those in 26 patients with severe idiopathic constipation (mean age 32 years, spontaneous bowel frequency less than one per week). In the patients there was a reduction in the follicular phase of progesterone (4.5 v 4 nmol/l, p = 0.006, median value, controls v patients), 17 hydroxyprogesterone (9.7 v 5.8 nmol/l, p = 0.01), cortisol (387 v 245 nmol/l, p = 0.008), testosterone (2.3 v 1.8 nmol/l, p less than 0.001), androstenedione (10.3 v 8.4 nmol/l, p = 0.02), and dehydroepiandrosterone sulphate (5.1 v 3.0 mumol/l, p = 0.03). In the luteal phase there was a reduction of oestradiol (483 v 350 pmol/l, p = 0.015), cortisol (322 v 242 nmol/l, p = 0.047), and testosterone (2.4 v 1.7 nmol/l, p = 0.003). The concentrations of sex hormone binding globulin, prolactin, luteinising hormone, and follicle stimulating hormone were not significantly different in either phase of the cycle. Women with severe idiopathic constipation have a consistent reduction in steroid hormones.