Objective : To evaluate factor XII deficiency in patients with recurrent spontaneous abortion and its relation to aPTT. Material and Method: Factor XII was analyzed by clotting method. Results: Of 70 patients with recurrent spontaneous abortion, there were 35 cases of factor XII deficiency. Among them, there were only 3 cases of prolonged aPTT. Conclusions: It is still unclear whether factor XII deficiency is related to recurrent spontaneous abortion. Molecular approaches should be used to understand further the causal relationship. But based on this result, in the workup of patients with recurrent spontaneous abortion, factor XII should be included. aPTT is not likely to represent the abnormality of factor XII.