Objective: To determine whether a new assisted hatching (AH) method increases the implantation and clinical pregnancy rates of frozen-thawed day-3 (D3) embryos. Design: Prospective study. Setting: A university hospital in vitro fertilization (IVF) program. Patient(s): Patients who had their first IVF/intracytoplasmic sperm injection (ICSI) cycles between June 1, 2006, and December 31, 2008, with fresh IVF-embryo transfer failures or without fresh embryo transfer. Intervention(s): The couples were randomized into thawed embryo transfer after AH versus no AH. In the AH group, the zona pellucida (ZP) of D3 frozen-thawed embryos was expanded by injected hydrostatic pressure after thawing. In the control group, embryos were pierced by ICSI needles without expanding the ZP. Main Outcome Measure(s): Clinical pregnancy and implantation rates. Result(s): The morphologic features of the blastomeres were carefully monitored and recorded. In the AH group, 244 embryos were thawed, and 178 (73.0%) survived; in the control group, 259 embryos were thawed, and 190 (73.4%) survived. Despite the transfer of a similar number of embryos, the AH group resulted in statistically significantly higher implantation and clinical pregnancy rates compared with the no AH group. Conclusion(s): Mechanically expanding the ZP of frozen-thawed D3 embryos with injected hydrostatic pressure after thawing increases the implantation rate compared with control embryos. Copyright © 2010 American Society for Reproductive Medicine, Published by Elsevier Inc.