Female pseudohermaphroditism is caused by several etiologies. Here we report a case of aromatase deficiency who showed ambiguous genitalia and maternal virilization during pregnancy. The mother had noticed her own virilization from 16 wk of gestation without androgen exposure and had low urinary estriol levels (5~10 μg/ml at 35 wk of gestation). At birth, the patient presented severe virilization (Prader V), and was assigned as a male with a micropenis and unpalpable testes but the patient had a normal female karyotype and a uterus and cystic ovaries found by magnetic resonance imaging. The patient had a increase in serum 17α-hydroxy progesterone levels (basal 4.9 → 37 ng/ml after a single 0.25 mg/m(2) infusion of ACTH), but the increase in adrenal androgen was not sufficient to virilize the external genitalia. Dehydroepiandrosterone, 17α-hydroxy pregnenolone and deoxycorticosterone were within the normal ranges. These findings suggested a diagnosis of nonadrenal female pseudohermaphroditism. From the clinical features and biochemical data, we endocrinologically diagnosed her as having an aromatase deficiency. The aromatase gene is now under investigation for definite diagnosis. We finally agreed that aromatase deficiency should be suspected when both the mother and the newborn have been virilized.