To evaluate the relationship between habitual abortion of unknown etiology and expression of leukemia inhibitory factor (LIF) gene in the decidua,the expression of LIF gene in the decidua were detected in 32 cases of habitual abortion of unknown etiology (abortion group) and 36 cases of normal pregnancy (control group) by reverse transcription-polymerase chain reaction (RT-PCR). The results showed that the level of LIF mRNA expression of the decidua in abortion group was 0.512+/-0.219, significantly lower than those in the control group (1.314+/-0.457, P<0.01). The findings indicated that the low expression of LIF gene in abortion group might be one of the causes of habitual abortion. Analysis on the LIF gene expression of decidua can be used as a index for diagnosis of the habitual abortion.