The effect of doubling the human menopausal gonadotrophin (HMG) dose in the same treatment cycle in which the ovarian response after 5 days of ovarian stimulation with 225 IU/day is 'low', has been evaluated in a prospective randomized study. Forty-six patients met the ultrasound and oestradiol criteria for enrollment in the study, one patient participated twice. In 22 patients treatment was continued with 225 IU HMG/day and in 25 patients the HMG dose was increased to 450 IU/day. No effect of doubling the HMG dose was found on the length of the ovarian stimulation, peak oestradiol values, number of follicles > or = 11 and > or = 14 mm in diameter respectively on ultrasound on the day of HCG administration, number of cancelled cycles, number of oocytes at follicular puncture and the number of patients with < or = 3 oocytes at retrieval. It is concluded that doubling the HMG dose in the course of an IVF treatment cycle is not effective in enhancing ovarian response in low responders. This is in accordance with current theories on follicular growth, which state that follicular recruitment occurs only in the late luteal and early follicular phase of the menstrual cycle.