The molecular mechanism of human embryo implantation is poorly understood. The role of MUC4 mucin, present in endometrial epithelium, has never been explored, and results obtained in animal studies strongly suggest a role in implantation. We investigated the role of MUC4 in human embryo implantation.We analysed the MUC4 variable number of tandem repeat (VNTR) polymorphism in three populations by Southern blot analysis: spontaneously fertile patients (C), infertile patients with repeated unexplained implantation failures after IVF (IF) and patients with a child after IVF (IVF-C). We found no differences in the size or allelic distribution of MUC4 VNTR between these three populations. We also examined, in IVF-C and IF groups, the endometrial expression of MUC4 mRNA as well as the expression of the MUC4 glycoprotein together with estrogen receptor (ER) and progesterone receptor (PR). No expression differences could be detected. However, we noticed a pattern of expression for MUC4 protein, which is limited to patches of cells in the luminal and glandular epithelium.We conclude that the different-sized MUC4 alleles do not interfere with implantation. The absence of coexpression of MUC4 and the steroid receptors suggests that MUC4 expression is not directly regulated by steroids.