OBJECTIVE: To investigate the effects of female age on in vitro maturation and fertilization of immature oocytes from controlled ovarian hyperstimulation (COH) in human IVF-ET program. METHOD: A total of 96 immature oocytes (GV & metaphase I) obtained from 40 cycles of IVF-ET (29 patients). The mean age of female patients was 31.8+/-3.1 years. Ovulation was triggered by urinary or recombinant hCG. Immature oocytes were cultured with YS medium containing 30% of patients' human follicular fluids, LH (1 IU/mL), FSH (1 IU/mL) and EGF (10 ng/mL), and then matured oocytes were fertilized by ICSI. In vitro maturation and fertilization of immature oocytes were analyzed according to age of female (< 34 or > or = 34 years). RESULTS: The maturation rate was similar between two groups (68% vs 64%). The fertilization rate of in?vitro-matured oocytes was higher in patients < 34 years old, but there was no statistical significance (64% vs 50%, p=0.347). The fertilization rate of in-vitro-matured oocytes was significantly lower compared with those of in-vivo-matured oocytes in both age groups (64% vs 79%, p=0.035, 50% vs 86%, p=0.007). CONCLUSION: In older female group, fertilization rate of in-vitro-matured oocytes seems to be decreased. Further investigations should be warranted to increase fertilization potential of in-vitro-matured oocytes.