Objective: To test the hypothesis that the concentration of early follicular phase serum mullerian-inhibiting substance (MIS) is associated with ovarian response in women undergoing ovulation induction in preparation for assisted reproductive technology (ART).Design: Retrospective analysis of frozen day 3 serum samples.Setting: Academic ART program.Patient(s): One sample of frozen day 3 serum from women with =<6 retrieved oocytes (n = 28) compared with women with =<11 oocytes retrieved (n = 79) in preparation for IVF.Intervention(s): None.Main Outcome Measure(s): Comparison of day 3 serum MIS levels between two groups of women. Other comparisons included maximum serum E 2 concentrations, number of retrieved oocytes, and percentage of mature oocytes between groups.Result(s): Mean serum MIS concentrations were 1.0 +/- 0.4 ng/mL compared with 2.5 +/- 0.3 ng/mL, or more than a 2.5-fold greater serum concentration of MIS in the group with =<11 oocytes retrieved compared with in the group with =<6 retrieved oocytes.Conclusion(s): These data demonstrate an association between early follicular phase serum MIS and the number of retrieved oocytes. Higher day 3 serum MIS concentrations were associated with greater number of retrieved oocytes.