In two separate prospective randomized series of 38 and 16 attempts of human in vitro fertilization (IVF), respectively, the fertilizing abilities of human spermatozoa capacitated in hyperosmotic media and in normosmotic medium were compared. Oocytes from each IVF attempt were divided into two equal groups and inseminated with spermatozoa incubated for 2 h in medium with an osmolality of 285 mosm/kg or in hyperosmotic medium of 315 (first experiment) or 345 mosm/kg (second experiment). The fertilizing capacity of the spermatozoa was not affected by incubation in medium of 315 mosm/kg as compared to spermatozoa incubated in medium of 285 mosm/kg. However, the fertilizing capacity of the spermatozoa incubated in medium of 345 mosm/kg was significantly reduced. The present study indicates that there is no advantage in the use of hyperosmotic media of 315 or 345 mosm/kg for capacitation of spermatozoa in human IVF.