The clinical and pathologic features of 13 cases of juvenile granulosa cell tumor were studied. Patients' ages ranged from 6 months to 56 years (median age, 17 years). Only one patient was postmenopausal. Three premenarchal patients had isosexual development. Five of seven postmenarchal patients had menstrual abnormalities, and two patients demonstrated virilization. Ascites was present in two patients. All patients had unilateral stage I tumors, ranging from 2.5 to 24.5 cm in greatest dimension (mean greatest dimension, 12.2 cm). Characteristic histologic features included nodular architecture, follicle formation, abundant interstitial and intrafollicular acid mucopolysaccharide-rich fluid, irregular microcysts, individual cell necrosis, and high mitotic activity (mean activity, 11 mitotic figures per ten high-power fields). The interstitial mucinous fluid consisted predominantly of hyaluronic acid. Immunohistochemical staining in five cases showed prominent positivity for vimentin (four cases), isolated cytokeratin AE1/3-positive cells (two cases), and nonreactivity for carcinoembryonic antigen and milk fat globule-2. Ultrastructurally, epithelial cells that resembled granulosa cells of the nonneoplastic preovulatory follicle and occasional cells with steroidogenic organelles were also found. Follow-up of ten patients revealed no tumor recurrences from six months to 33 years (mean, 9.5 years) after operation.