Adult granulosa cell tumors (GCTs) are the most common type of ovarian sex cord tumors. Menstrual irregularity, menorrhagia, or even secondary amenorrhea is frequently observed in premenopausal women bearing GCTs with hormonal activity. We report herein a case of GCT in a patient presenting with secondary amenorrhea and serum luteinizing hormone elevation. A 28-year-old primigravid Japanese woman was admitted complaining of secondary amenorrhea of 2 years' duration. Pelvic examination, transvaginal ultrasonography, and magnetic resonance imaging demonstrated a left ovarian tumor 4 cm in diameter. Serum hormone assays revealed a follicle-stimulating hormone level of 4.8 mIU/ml, luteinizing hormone (LH) of 35.8 mIU/ml, estradiol of 24 pg/ml, progesterone of 1.6 ng/ml, and testosterone of 40 ng/dl. A left salpingo-oophorectomy was performed. The tumor was diagnosed as an adult-type GCT stage IIb (FIGO [International Federation of Obstetricians and Gynecologists], 1988). Spontaneous menstruation occurred soon after the surgery. Serum levels of LH also decreased to normal levels and showed cyclic changes during the menstrual cycle. Subsequently, the patient conceived and delivered a healthy female baby. The tumor recurred in the pelvis 50 months after the initial conservative surgery, with elevated serum LH levels of 36.0 mIU/ml and amenorrhea. The patient was treated by hysterectomy, right salpingo-oophorectomy, omentectomy, paraaortic and pelvic lymphadenectomy, and low anterior resection of the recto-sigmoid colon. Her hormone levels progressed to the postmenopausal state after this surgery. Although LH elevation in patients with GCT is rare and its mechanism is unknown, monitoring of serum LH may provide an additional tumor marker after conservative surgery in such patients.