From 1993 to 1994, in our centre, laser-assisted hatching was performed on 2- to 4-cell stage embryos obtained from in-vitro fertilization (IVF) patients. We treated 376 embryos from 96 patients with repeated IVF failures (two to four attempts) (group A) and 397 embryos from 111 patients undergoing IVF for the first time (group B). Embryos were transferred immediately after the laser treatment. Both groups were compared to control groups (A' and B') whose embryos were transferred with intact zona pellucida (ZP). The resulting clinical pregnancies were 41 in A and 44 in B versus 24 in A' and 23 in B' respectively. The pregnancy rates per patient were 42.7 and 39.6% versus 23.1 and 19% in the control groups (P < or = 0.05), while the implantation rates per embryo were 12.2% in A and 11.8% in B versus 7.3% and 7.1%. These results show that laser zona thinning of human embryos at 48 h after egg retrieval significantly increases the implantation and pregnancy rate (P < or = 0.05).