Becker nevus syndrome is the association of becker``s nevus with breast hypoplasia and other ipsilateral bone or muscle changes. It is considered to be a hormone-dependent disorder caused by a disturbance in the activity of the androgen receptor. The receptor appears to be increased in Becker``s nevus, which may influence to the development of associated lesions. Herein, we report a case of Becker nevus syndrome improved after the treatment with spironolactone. A 13-year-old girl presented with well-defined brownish patch over left breast for 8 years. In addition, she noticed hypoplasia of her ipsilateral breast (12 x 10 cm) since 10 years age. Histopathologic examinations revealed acanthosis, elongation of rete ridges in epidermis and hyperpigmentation of the basal epidermal layer. The diagnosis has been confirmed to Becker``s nevus associated with ipsilateral breast hypoplasia. The Laboratory test (CBC, electrolytes, and LH/FSH) done and showed normal range. Her hypoplastic breast was treated with spironolactone with 50mg single dosage twice daily. She experienced mild menstrual irregularity and it was improved within 1 month. The left breast had enlarged abruptly to 13 x 10.5 cm after the treatment of 19 weeks.