Objective: To demonstrate the effects of standard sperm parameters and sperm motility on fertilization rate and embryo development. Design: Prospective clinical study. Setting: Istanbul University IVF-ET Research and Practice Center, Istanbul - Turkey. Patients: Two hundred-nine patients who underwent IVF-ET program in 1995. Main Outcome Measures: Standard sperm parameters such as sperm concentration, morphology and motility were determined prior and after the swim-up procedure. Sperm motility 24 hours after oocyte insemination was also determined. Results: Sperm concentration, motility and morphology in the fresh semen preparation, sperm concentration in the swim-up inseminate and sperm motility 24 hours after oocyte insemination were all found to be significant parameters effecting fertilization and embryo development (p<0.001). In order to determine independent prognostic parameters effecting fertilization and embryo development, a logistic regression analysis was employed, including the standard sperm parameters before and after swim-up, sperm motility 24 hours after oocyte insemination and number of oocytes picked up at IVF-ET program. Sperm motility 24 hours after oocyte insemination was found to be the significant parameter influencing fertilization and embryo development (p<0.001). The cut-off level for sperm motility at 24 hours post-insemination was calculated as 50% by student t test. Eighty percent of the patients- having more than 50% sperm motility at 24 hours after the insemination had achieved fertilization, whereas only 4.2% of the patients having less than 30% motility had achieved fertilization. Conclusion: The evaluation of sperm motility at 24 hours after oocyte insemination is a simple and readily performed test which can provide valuable information regarding outcome of conventional IVF-ET.