Received August 30, 2002
Accepted September 12, 2002

Purpose: The objective of this study was to evaluate the efficacy of one laser-zona thinning (L-ZT) microdissection applied to thawed embryos transfers (ETs) compared to thawed ETs not submitted to L-ZT. Methods: A total of 135 patients who were submitted to an ICSI program and had their excess embryos frozen and thawed by the slow method were divided, in a prospective and randomized manner(2:I), into two groups at the time of receiving their thawed embryos:Group I:(90) patients whose thawed embryos transferred were all submitted to L-ZT; Group II:(45) patients whose thawed embryos transferred were not submitted to L-ZT (control group). Results: ZP thickness of group I embryos(17.3±2.8µm) did not differ (p=0.4) from that of group II embryos(17.1±2.4µm). There were no significant differences between groups I and II with respect to the implantation rates (%) in the natural (group I=11.7%; group II=12.5%; p=1) or substitutive (group I = 26.5%; group II=20%) cycle. The clinical pregnancy rate per transfer of group I (26.6%) did not differ significantly (p=0.5) from that of group 11 (20%). Conclusions:Pregnancy and implantation rates were not different between the group submitted to L-ZT and the control group. ln the present study, the use of L-ZT was not effective for frozen-thawed embryos. However. this conclusion is limited to a situation in which length of the laser zona thinning was =30µm.