Aromatase deficiency is a rare autosomal recessive disorder in which individuals cannot synthesize endogenous estrogens. If a fetus lacks aromatase activity, dehydroepiandrosterone sulfate produced by the fetal adrenal glands cannot be converted to estrogen by the placenta, and is converted to testosterone peripherally and results in virilization of both fetus and mother. Virilization manifests as pseudohermaphroditism in female infants, with hirsutism and acne in the mother; the maternal indicators resolve following delivery. Affected females are usually diagnosed at birth because of the pseudohermaphroditism. Cystic ovaries and delayed bone maturation can occur during childhood and adolescence in these girls, who present at puberty with primary amenorrhea, failure of breast development, virilization, and hypergonadotropic hypogonadism. Affected males do not present with obvious defects at birth. Their clinical symptoms include tall stature, delayed skeletal maturation, delayed epiphyseal closure, bone pain, eunuchoid body proportions, and excess adiposity. Estrogen replacement therapy reverses the symptoms in males and females (summary by Jones et al., 2007).