Leydig cell hypoplasia(LCH) or agenesis, which has an autosomal recessive inheritance pattern, is a well defined form of male pseudohermaphroditism resulting from inadequate fetal testicular Leydig cell differentiation. These patients exhibit a wide clinical spectrum that ranges from phenotypic females to males with micropenis, and have low levels of serum testosterone and elevated levels of LH. Recently, it has become clear that defective male sex differentiation in patients with LCH is caused by deficient LH receptor signal transduction. The fetal Leydig cells of LCH patients fail to respond to adequate levels of placental hCG, resulting in inappropriate fetal androgen production. At puberty, adult-type Leydig cells do not mature and do not produce sufficient androgen for normal secondary sex differentiation.