The association between blocking folate receptor (FR) autoantibodies and subfertility was investigated in a longitudinal study of women attempting to become pregnant. Seventeen women with subfertility (failure to conceive during 12 menstrual cycles) and 25 control women (women who conceived and went on to have normal pregnancy outcomes) were studied. Subfertility risk was 12 times higher in women with blocking FR autoantibodies compared with those without (odds ratio, 12; 95% confidence interval, 1.9-129.6).