Serum hormone concentrations were determined at intervals during the last 17 days of the menstrual cycle in 35 patients with premenstrual tension (PMT) and 11 control subjects without symptoms. The maximum mean concentration of oestradiol occurred 17 days before menstruation in the patients and 14 days before in the controls. The maximum concentrations of progesterone were similar in the two groups but the mean concentrations rose earlier in the cycle in the patients with PMT. These results suggested that the patients tended to ovulate earlier in the cycle than the controls and on the basis of the ovulatory surge in gonadotrophins two groups could be identified, group A who showed signs of ovulation 14 days or less before menstruation (17 patients, 9 controls) and group B who ovulated more than 14 days before menstruation (18 patients, 2 controls). There were no significant differences between the groups in prolactin, thyroid stimulating hormone or testosterone levels, but cortisol concentrations were uniformly higher in both groups of patients compared with those in the controls. Follicular growth was assessed with ultrasound in 18 patients and 16 control subjects. Mean follicular diameters were significantly lower in the patients than in the control group at the time of ovulation. Oestradiol determinations done at the same time correlated with the diameters and were also significantly lower in the patient group. The results suggest that ovulation tends to occur prematurely in women with PMT.