The present study was designed to investigate if antithyroid antibodies (ATA) could affect the pregnancy outcome in euthyroid women undergoing in vitro fertilization and embryo transfer (IVF-ET). From October 1995 to September 1996, 28 euthyroid women with ATA who underwent IVF-ET were studied. Fifty-one euthyroid women without ATA who underwent IVF-ET served as control. Thyroid peroxidase antibody (TPOA) and thyroglobulin antibody (TGA) were assayed using radio ligand assay kits as ATA. All patients included in study and control groups had only tubal factor in infertility. Long protocol of gonadotropin-releasing hormone agonist (GnRH-a) was used for controlled ovarian hyperstimulation (COH) in all patients. There were no significant differences between study and control groups in patient characteristics such as age, infertility duration and hormonal profile. There were also no significant differences between two groups with respect to the clinical response to COH and IVF results such as number of retrieved oocytes, fertilization rate, number of embryos frozen and number of embryos transfered. There were no correlations between ATA (TPOA and TGA) titers and fertilization rate. The clinical pregnancy rate per cycle seemed to be lower in the study group than in the control group (26.3% vs 39.3%), but the difference was not statistically significant. The biochemical pregnancy rate per cycle and miscarriage rate were significantly higher in the study group at 18.4% (7/38) and 40.0% (4/10) compared with 5.6% (5/89) and 11.4% (4/35) in the control group. In the study group, both TPOA and TGA titers were significantly higher in the biochemical pregnancy group than in the clinical pregnancy group or non-pregnancy group. In 10 women with ATA who achieved pregnancy following IVF-ET, both TPOA and TGA titers were significantly higher in the miscarriage group than in the ongoing or delivery group. In conclusion, euthyroid women with ATA appear to represent a less favorable subset within other tubal factor patients when treated with IVF-ET.