In women having inactive ovaries, controlled preparation of the endometrium has been achieved with exogenous oestradiol and progesterone. We report on the feasibility and practicality of using a similar regimen for timing transfers of cryopreserved embryos in women whose ovaries have not been suppressed. A total of 91 women having cryopreserved embryos from previous in-vitro fertilization (IVF) attempts received 4 mg/day of oestradiol valerate, starting on cycle day 1 of spontaneous (n = 85) or induced (n = 6) menstruation. A single blood sample was obtained on cycle day 14 for the measurement of plasma progesterone, oestradiol and luteinizing hormone (LH). Vaginal administration of micronized progesterone (300 mg/day) was started on day 15. Cryopreserved embryos were transferred on day 17 or 18 provided that day 14 plasma progesterone remained < or = 0.5 ng/ml, thereby confirming the absence of spontaneous ovulation prior to the administration of exogenous progesterone. Out of 91 cycles studied, plasma progesterone was found to be elevated (> 1 ng/ml) in only three (3.2%). Of the 88 scheduled transfers, 31 did not take place because no embryo survived thawing. In the remaining 57 cycles, 116 embryos were transferred resulting in 10 pregnancies, giving pregnancy and embryo implantation rates of 17.5 and 8.6% respectively. When a positive beta human chorionic gonadotrophin (HCG) titre was obtained, supplementation with oral oestradiol and vaginal progesterone was continued until placental autonomy was achieved. Of the 10 pregnancies, five (50%) were lost during the first trimester (biochemical, n = 1; miscarriage, n = 3; ectopic, n = 1).(ABSTRACT TRUNCATED AT 250 WORDS)