Objectives: Several investigators reported that antiphospholipid antibodies (aPL) are associated with infertility. These findings have led clinicians to treat infertile women with aPL and undergoing IVF in an effort to improve reproductive outcome. Our objectives were: (1) To determine if aPL are more common in infertile women undergoing IVF compared to fertile controls and compared to women with antiphospholipid syndrome (APS), and (2) to examine the relationship between aPL and implantation rates in IVF patients.Methods: Blinded serum samples from 166 infertile women undergoing IVF, 158 healthy, fertile controls, and 50 women with APS were assayed for antibodies against seven phospholipids (cardiolipin, phosphatidylserine, phosphatidic acid, phosphatidylinositol, phosphatidylglycerol, phosphatidylcholine (aPC), and phosphatidylethanolamine (aPE)). Results were normalized using positive anticardiolipin and assay-specific standards. Antibody tests were considered positive if greater than the 99 t h percentile of 169 blood bank controls. Mean optical density (OD) ratios for each antibody were compared using ANOVA and the proportion of positive patients were compared using chi-square.Results: The proportion of infertile women undergoing IVF and fertile controls that were positive for any aPL were similar (28% vs 20%; P = .20). In addition, mean OD ratios were similar for all aPL except aPC in women undergoing IVF and fertile controls. In contrast, mean OD levels and the proportion of women testing positive for aPL were higher in women with APS compared to infertility patients and fertile controls. The successful implantation rate did not differ between women with or without aPL (61 vs 55%; P = .46). 30% of those who achieved successful implantation were positive for any aPL compared to 25% of those who did not (P = 0.49).Conclusions: Infertile women undergoing IVF are no more likely to have a positive test for aPL than fertile controls. The successful implantation rate does not differ between women who have and do not have aPL. Our data do not support testing women with infertility for aPL.