High-frequency uterine contractions (UC) at the time of embryo transfer have been shown to hamper the outcome of in-vitro fertilization (IVF). As UC are postulated to be hormone-regulated, we aimed to investigate the role of plasma oestradiol and progesterone concentrations on UC during ovarian stimulation for IVF. A total of 59 women were studied on the day of administration of human chorionic gonadotrophin (HCG) and embryo transfer. Plasma oestradiol and progesterone concentrations were measured, and 5 min ultrasound scans of the uterus were digitized with an image analysis system to assess UC frequency and direction. Cycles were sorted according to whether progesterone concentrations on the day of embryo transfer were < or =100 (n = 34) or >100 (n = 25) ng/ml. On the day of HCG, UC frequency was similar in both groups at 4.5+/-0.2 and 4.6+/-0.3 UC/min (mean +/- SE) respectively. On the day of embryo transfer, UC frequency remained steady in the low progesterone group, whereas it decreased (3.5+/-0.2 UC/min) in the high progesterone group (P < 0.001), and was negatively correlated with progesterone concentrations (r = -0.56; P < 0.001). No influence of oestradiol on UC was noticed. These observations confirm the utero-relaxing effects of progesterone in the non-pregnant uterus and support the administration of progesterone before embryo transfer to increase tissue concentrations and improve the outcome of IVF.