Recently, the role of IL-10, in controlling the response of the maternal immune system to the embryo has been highlighted. We studied the expression of IL-10 by immunohistochemistry in human endometrium of patients with habitual abortions and in fertile controls. Endometria of 79 women affected by habitual abortion, without evidence of known abortive causes and sampled in the late secretory phase, were studied by immunohistochemistry for IL-10 and CD-56 expression. Monoclonal antibodies for these antigens were used on paraffin embedded tissues. Controls were endometria of 22 fertile women and 10 IVF pregnancies. HSCORE system was used for a semi-quantitative assay. Stromal cells CD56 positive of secretory phase endometria and the decidua of pregnancies express IL-10, whereas no expression was observed in proliferative phase samples. In the samples of habitual aborters, a statistical significant decrease of IL-10 expression was shown. 29 of 79 patients showed a reduction in IL-10 expression of more than 50%, relative to controls. IL-10 is expressed in stromal cells of secretory phase and pregnancy, and markedly reduced in habitual aborters. Assessment of IL-10 in secretory endometrium may be used in these patients to find Th2 impairment.