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 less than or equal to6 retrieved oocytes (n = 28) compared with women with less than or equal to11 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, 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 greater than or equal to11 oocytes retrieved compared with in the group with 56 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. (Fertil Steril(R) 2002 77:468-77. (C)2002 by American Society for Reproductive Medicine.).