PURPOSE:The aim of this study was to evaluate the efficacy of intracytoplasmic sperm injection (ICSI) in comparison with conventional reinsemination using fertilization failed oocytes by conventional in vitro fertilization (IVF). METHODS:Oocytes were collected from patients of IVF or ICSI cycles. Patients were grouped by fertilization techniques: group 1: conventional IVF; group 2: reinsemination after conventional IVF failure; group 3: regular ICSI; group 4: 1-day-old ICSI after conventional IVF failure; group 5: 2-day-old ICSI after conventional IVF failure; group 6: re-ICSI after regular ICSI failure. RESULTS:In different insemination groups, normal fertilization rate was higher (P < 0.001) in 1-day-old ICSI (47.1%) and 2-day-old ICSI groups (40.0%) than in reinsemination (14.7%). Abnormal fertilization rate was higher (P < 0.05) in re-ICSI group (21.7%) than any other groups (range: 0-8%). Cleavage rate was higher in 1-day-old (36.7%) and 2-day-old ICSI groups (36.0%) than in reinsemination (5.3%, P < 0.001) or re-ICSI groups (17.4%, P < 0.05). Pregnancy rate was 27.6% and 20.0% in conventional IVF and regular ICSI groups, respectively. However, 1-day-old ICSI (group 4) and 2-day-old ICSI (group 5) were attempted once embryo transfer (ET) but failed pregnancy occurred in each group. CONCLUSIONS:In fertilization failure cycles, late ICSI increases the rate of fertilization and embryonic development and may rescue the completely failed attempt of pregnancy.