In spite of much progress in vitro fertilization and embryo transfer (IVF-ET) program, the pregnancy rate remains at 20-30%, and the endometrial implantation rate per embryo transferred at 10-15%. As a result, about 90% of embryos may fail to implant to the endometrium, and many attempts such as optimization of follicular development, improvement of in vitro culture system including coculture, and micromanipulation of zona pellucida have been made to improve embryonic implantation after IVF-ET. Recently, several procedures of assisted hatching (AH) using micromanipulation have been introduced, and pregnancies and births have been obtained after AH. To develop and establish AH as an effective procedure to improve embryonic implantation, AH with partial zona dissection (PZD) was performed in 116 cycles of 89 infertile couples who had previous repeated failures of standard IVF-ET more than two times (Group I: 71 cycles in 54 patients), or who had implantation failure of embryos with good quality (Group II: 15 cycles in 13), or who had undergone AH without specific indication (Group III: 30 cycles in 22) from January, 1995 to Februry, 1996, and the outcomes of AH were analyzed according to pregnancy rate. The number of oocytes retrieved after controlled ovarian hyperstimulation (COH) was $9.9{pm}7.1$ in Group I, $11.5{pm}4.5$ in Group II, and $7.9{pm}6.4$ in Group III. The number of embryos transferred after AH was $4.7{pm}1.8$ in Group I, $5.3{pm}1.3$ in Group II, and $3.5{pm}2.4$ in Group III. The mean cumulative embryo score (CES) was $56.8{pm}30.0$ in Group I, $76.1{pm}35.9$ in Group II, and $38.5{pm}29.9$ in Group III. The overall clinical pregnancy rate per cycle and per patient was 12.7% (9/71) and 16.7% (9/54) in Group I, 33.3% (5/15) and 38.5% (5/13) in Group II, and 6.7% (2/30) and 9.1% (2/22) in Group III, respectively. There were significant differences in the numbers of oocytes retrieved and embryos transferred, CES, and the clinical pregnancy rate per cycle among three groups. There was a significant inverse correlation between basal serum FSH level and CES, and no pregnancy occurred in patients with CES less than 20. In conclusion, AH of human embryos with PZD prior to ET has improved the implantation and pregnancy rates in IVF-ET patients with the past history of repeated failures, especially in spite of transfer of embryos with good quality, and AH will provide a range of novel techniques which may contribute much to effective management of infertile couples.