Efficient gonadotrophin therapy requires both biochemical and ultrasonic measurements of ovarian response. Biochemical monitoring can take the form of measurement of plasma oestradiol levels or urinary oestrogen excretion. Levels of first morning urinary oestrogen to creatinine ratio have been shown to correlate well with 24-hour total urinary oestrogen excretion. Such urine collection is not cumbersome and is less invasive than daily venepuncture. Eighty gonadotrophin treatment cycles were evaluated by first morning urinary oestrogen to creatinine ratio and ultrasound scanning of follicles. Analysis confirmed that the ratio correlated significantly both with plasma oestradiol and 24-hour urinary oestrogen levels. The administration of the ovulating dose of human chorionic gonadotrophin depended primarily on ultrasonic demonstration of the presence of mature follicles in the ovaries. There was no increased incidence of multiple gestation or hyperstimulation syndrome in 135 treatment cycles. It is concluded that first morning urinary oestrogen to creatinine ratio is a convenient and cheap method of efficient biochemical monitoring of ovarian response to gonadotrophin therapy.