Importance of 17-β estradiol and endometrial thickness in assisted reproductive cycles with embryo transfer after oocyte thawing. Interest in oocyte cryopreservation has recently increased, especially in Italy following the introduction of the Law 40 of February 19th 2004 that forbids human embryo experimentation. Today, biological research is concentrated in new protocols application to improve survival and fertilization rates of oocytes; nevertheless, the clinical efficiency of cryopreservation method is still controversial. By this way, to perform embryo transfer at correct endometrial timing could represent an essential viable of intervention. 152 oocytes were frozen by slow cooling protocol (0.1M Sucrose); 64 of these were thawed by fast thawing procedure (0.2M Sucrose) and 13 embryo transfer were performed in women having mean age 33.8 years. 609 oocytes were frozen by slow cooling protocol (0.3M Sucrose); 64 of these were thawed by fast thawing procedure (0.3M Sucrose) and 38 embryo transfer were performed in women having mean age 34.8 years . Endometrial growth was obtained by estrogenic oral or transdermic administration and intramuscular progestinic therapy and was observed by ultrasound transvaginal scanning and hormonal haematic assays (17-β estradiol). Only patients having endometrialthickness ≥ 8 mm and 17 β-estradiol value > 200 mU/ml were suitable for embryo transfer. In the first group any pregnancy was observed while in the second 7 pregnancies were obtained (corresponding to 13.7% pregnancy rate/transfer with 1.6 embryos transferred for each couple). Four of these pregnancies resulted in spontaneous abortions in the first trimester of gestation while the others give rise to birth of 3 females; these are their respective clinical values: weight: 4175 g, 3430 g, 2790 g; pregnancy timing: 42 weeks , 40 weeks, 39 weeks; way of delivery: natural induced, caesarean birth, caesarean birth; apgar index: 10, 10, 10. Our data show that 0.3M Sucrose protocol, in comparison to 0.1M Sucrose protocol, don’t improve oocyte survival rates (73.4% vs 73.8%), and even it increase oocyte fertilization rate (51.2% vs 70.2%) and embrional cleavage (59.6% vs 75.7%). On therapeutic profile, oral administration is better tolerated and it assure steady release in blood circulation than transdermic therapy however, both give rise to same experimental results. Endometrial growth is proportional to plasmic estrogenic improve in 63/70 cycles (90%) for reaching mean value of 9.6 mm in XIV day with 17-β estradiol serum value of 700 pg/ml. These values correlate with a good term pregnancy rates (66%), while major 17-β estradiol (1000-2400 pg/ml) serum value correspond to higher abortions percentage (75%). Our study indicate that slow cooling/fast thawing 0.3M Sucrose protocol is still experimental for low implantation rates and for small number of pregnancies worldwide, so we cannot confirm it ensures a high standard of clinical efficiency. Nevertheless, this procedure, in association with endometrial timing study and embryo transfer best conditions, could give possibilities to increase embryo implantation rate in the in vitro fertilization cycles, expecially in Italy where embryo cryopreservation is forbidden. Today these techniques are the only available while we wait for modification of Law about medical assisted reproduction.