OBJECTIVE: To investigate the effect of short-term glucocorticoid administration on embryotoxicity of sera from infertile patients with mild to moderate endometriosis. DESIGN: Prospective longitudinal study. SETTING, PATIENTS: Eight infertile patients with mild to moderate endometriosis and a control group of eight infertile patients with tubal infertility were selected on the basis of laparoscopic examination. INTERVENTIONS: Basal (B) serum collection and day 1 (T1), day 3 (T2), day 6 (T3), and day 12 (T4) serum drawn after a 3-day glucocorticoid treatment in endometriosis patients. MAIN OUTCOME MEASURE: Embryotoxicity of endometriosis sera, before and after glucocorticoid treatment, was investigated using a bioassay performed on two-cell mouse embryos. Interleukin 1 alpha (IL-1 alpha) and antismooth muscle, antimitochondrial, and antinuclear autoantibodies were also tested in these sera. RESULTS: At 50% concentration, endometriosis serum is embryotoxic in comparison with control; 0% versus 61% of the embryos reached the blastocyst stage at 72 hours, respectively (basal versus control, P less than 0.001). However, this embryotoxicity significantly decreases 12 days after glucocorticoid treatment in comparison with untreated sera; 32.4% versus 0% of the embryos reached blastocyst stage at 72 hours, respectively (T4 versus basal, P less than 0.001), although they did not reach nontoxic levels (greater than 50%). Interleukin 1 alpha was undetectable in all samples analyzed. In endometriosis sera, antismooth muscle antibody was detected. CONCLUSIONS: At 50% concentration, serum from infertile patients with minimal to moderate endometriosis appears to be embryotoxic to the in vitro development of two-cell mouse embryos. However, this embryotoxicity significantly decreases 12 days after a 3-day treatment with glucocorticoids.