Müllerian inhibiting substance (MIS) is a sexually dimorphic gonadal hormone with proven efficacy in the evaluation of boys with cryptorchidism and children with intersex conditions. We examined the role of MIS determination in the evaluation of 65 phenotypic females with mild virilization. Among the 28 subjects with MIS values elevated above the normal female range, all had abnormal gonadal tissue: ovotestes in 11, testes in 7, dysgenetic gonads in 7, and MIS-secreting ovarian tumors in 3. Among the 37 children with serum MIS in the normal female range, 19 had detectable MIS and 18 had unmeasurable MIS. In the former group with measurable but normal female MIS values, 16 subjects had ovaries, 1 had an ovotestis, and 1 had dysgenetic gonads containing testicular elements. Of 18 children with undetectable MIS values, 16 had ovaries and 2 had ovarian dysgenesis. In this study, elevation of serum MIS above the normal female range was consistently associated with the presence of testicular tissue or MIS- secreting tumors, mandating additional evaluation and surgical exploration. A value within the normal female range in a virilized patient did not exclude dysgenetic testicular tissue or ovotestis, whereas undetectable values were consistent with the absence of testicular tissue.