Fetal male sexual differentiation is driven by two testicular hormones: testosterone (synthesized by interstitial Leydig cells) and antimüllerian hormone (AMH; produced by Sertoli cells present in the seminiferous tubules). Intersex states result either from gonadal dysgenesis, in which both Leydig and Sertoli cell populations are affected, or from impaired secretion or action of either testosterone or AMH. Until now, only Leydig cell function has been assessed in children with ambiguous genitalia, by means of testosterone assay. To determine whether serum AMH would help in the diagnosis of intersex conditions, we assayed serum AMH levels in 107 patients with ambiguous genitalia of various etiologies. In XY patients, AMH was low when the intersex condition was caused by abnormal testicular determination (including pure and partial gonadal dysgenesis) but was normal or elevated in patients with impaired testosterone secretion, whereas serum testosterone was low in both groups. AMH was also elevated during the first year of life and at puberty in intersex states caused by androgen insensitivity. In 46,XX patients with a normal male phenotype or ambiguous genitalia, in whom the diagnosis of female pseudohermaphroditism had been excluded, serum AMH levels higher than 75 pmol/L were indicative of the presence of testicular tissue and correlated with the mass of functional testicular parenchyma. In conclusion, serum AMH determination is a powerful tool to assess Sertoli cell function in children with intersex states, and it helps to distinguish between defects of male sexual differentiation caused by abnormal testicular determination and those resulting from isolated impairment of testosterone secretion or action.