Background: Leukemia inhibitory factor (LIF) is a group of secreted glycoproteins with molecular weights ranging from 38-67 kD, resulting from differential protein glycosylation. LIF is constitutively expressed at high levels in the human fallopian tube epithelium and has an important role in the motility and vitality of sperm. In the present study, the effect of human recombinant LIF on human sperm motility and survival in vitro was investigated.Methods: Normal spermatozoa of 30 fertile men were collected and after preparation were incubated in Ham's F10+FCS 10% medium, containing various concentrations (0, 3, 5, 10, and 50 ng/ml) of LIF at 37 ºC under 5% CO2 for 6, 24 and 48 hours. Sperm motion characteristics were measured using a Makler chamber. Sperm survival was determined using the hypoosmotic swelling test. Collected data were analyzed by one-way ANOVA and LSD test using SPSS version 11. The difference in values were considered significant when p<0.05.Results: Sperm motility was significantly higher after 24 h exposure to 5-10 ng/ml LIF (p<0.05). The survival rate of sperm was significantly prolonged when exposed to 50 ng/ml LIF (p<0.05). Nonprogressive motility and survival rate of sperm were significantly higher after 48 h exposure to 50 ng/ml and 10-50 ng/ml LIF, respectively. (p<0.05). There was no significant difference in progressive sperm motility during the 48 h exposure of sperm to the various concentrations of LIF.Conclusion: According to our results, the effect of LIF on sperm motility and survival were dependent on the dose of LIF supplementation and the length of incubation."n