We report the clinicopathologic findings of an unusual case of adult granulosa cell tumor with androgenic manifestation in a 13-year-old prepubertal girl. The patient had never had a menstrual period and presented with a 1 year history of hirsutism. Physical examination was only remarkable for an increase in facial and abdominal hair, both with a male pattern of distribution. A pelvic ultrasound demonstrated a 6.0 cm right adnexal cyst. Plasma testosterone and 17-hydroxyprogesterone levels were elevated. The patient initially was treated with monophasic oral contraceptive pills for 3 months and owing to lack of response, she underwent an exploratory laparotomy in which a left ovarian tumor, 7.0 cm in greatest dimension, and a 6.5 cm right paratubal cyst were found. A wedge biopsy of the left ovary and subsequent left oophorectomy with right salpingectomy were performed. No gross evidence of disease outside the ovary was noted. Microscopic examination of the left ovarian tumor revealed the typical features of an adult granulosa cell tumor. No tumor was seen outside the ovary. Six days after surgery, plasma testosterone and 17-hydroxyprogesterone levels were in the normal range. Nine months postoperatively, the patient shows no evidence of disease. To our knowledge, this represents the first case of a prepubertal patient with an adult granulosa cell tumor with androgenic manifestations reported in the English literature.