Twenty-eight patients superovulated with clomiphene citrate (CC) and human menopausal gonadotrophin (HMG) were given a single injection of 25 mg progesterone (P group) 4 h prior to the ovulation-indudng injection of human chorionic gonadotrophin (HCG). Plasma and urinary LH levels were significantly higher (P < 0.05) in the P group immediately prior to HCG compared to controls. Plasma progesterone concentrations were also elevated (P < 0.01) in the P group from the time of injection to oocyte recovery. The number of mature oocytes recovered was also higher (P < 0.001; 59% versus 40% in controls) and the time interval between oocyte recovery and insemination was also shorter (P < 0.01) in the P group. The pregnancy rate/replacement 15 days after oocyte recovery was 39% versus 23% in the P and control groups respectively. It was concluded that as more mature oocytes were recovered in the P group, progesterone supplementation in the late follicular phase may be beneficial for patients undergoing GIFT. This was borne out when the first two GIFT patients pretreated in this way became pregnant.