Among other causes of hyperestrogenism, granulosa cell tumors (GCT) can be the cause of excessive estrogen production. However, localizing the source of hyperestrogenism preoperatively can be difficult when the tumor is minuscule. The authors describe herein a case of microscopic GCT associated with postmenopausal vaginal bleeding and marked breast enlargement. Extensive work-up included endocrinological tests, pelvic ultrasound, computed tomography, and magnetic resonance imaging, without direct localization of the source of hyperandrogenism. Persistently high plasma estradiol prompted an explorative laparoscopy. Intraoperative selective ovarian venous sampling revealed a 13-fold higher estradiol concentration in the left ovarian veins. Based on these findings, a left salpingooophorectomy was performed, but failed to identify estrogen-producing lesions on frozen sections. The detailed histological and immunohistological examination confirmed the diagnosis of a GCT of 5 mm in diameter, postoperatively. Exploratory laparoscopy with intraoperative selective blood sampling of the ovarian veins is a useful approach in patients without accurate preoperative localization of diminutive estrogen-producing tumors of the ovaries.