In man and some animals regulation of embryo implantation by endometrial expression of the highly polymorphic MUC 1 mucin has been suggested. We assessed the polymorphism of MUC 1 in women known to be fertile and those with infertility due to suspected failure of embryo implantation. The median of the lower allele size in the infertile group was only 2–5 kb compared with 3·4 kb in the fertile group (p=0·0029, difference 0–9, [95% Cl 0·1·13]). Women with unexplained infertility might have a genetic susceptibility to failure of embryo implantation due to small MUC 1 allele size.