['To determine whether fertilization rate serves as a biological assay, reflects oocyte quality, and may be used to help predict patient implantation rate.', 'Retrospective cohort study.', 'Academic center.', 'Couples undergoing 3603 in vitro fertilization (IVF) cycles from 2001 to 2007.', 'None.', 'We compared the implantation rate among cycles with high versus low fertilization rate. Univariate analyses were performed to determine the association of implantation rate with potential confounding variables: age, day-3 follicle-stimulating hormone level, day-3 estradiol level, antral follicle count, oocyte number, cycle attempts, embryo grading, and number of embryos transferred. Multivariate analysis was then performed to determine whether the fertilization rate remained an independent predictor.', 'Cutoffs for fertilization rate were 50% for intracytoplasmic sperm injection (ICSI) and 75% for conventional insemination. Higher ICSI fertilization was statistically significantly associated with the implantation rate (25.2% vs. 17.8 %). After adjusting for variables associated with implantation rate, fertilization rate for ICSI remained a strong independent predictor of implantation. Higher conventional insemination fertilization was statistically significantly associated with implantation (32.1% vs. 25.7%) and remained a statistically significant predictor after adjustment.'] Is fertilization rate an independent predictor of implantation rate?