The objective of this prospective randomized double-blind clinical trial was to evaluate whether laser zona pellucida thinning of human embryos improves clinical outcomes in women <or=37 years old undergoing IVF-ET treatment. The study did not reveal any significant beneficial effect of laser zona thinning on clinical pregnancy (16 out of 45 vs. 18 out of 39) and live birth rates (13 out of 45 vs. 16 out of 39) between the laser-treated and nontreated groups of patients, although there was a trend toward an increased incidence of dichorionic multiple pregnancies in the study group compared with the control group (7 out of 13 vs. 4 out of 16).