OBJECTIVE To study the clinicopathologic features, diagnostic criteria and prognostic parameters of juvenile granulosa cell tumor of ovary. METHODS The clinical and pathologic findings of 7 cases of juvenile granulosa cell tumor were retrospectively reviewed. Immunohistochemical study was carried out in 6 of these cases. The follow-up data were also analyzed. RESULTS The mean age of the patients was 24 years (range=6 to 53 years). Four patients presented with hormonal disturbance, while 3 patients presented with abdominal pain or swelling. Six patients underwent unilateral salpingo-oophorectomy. Six cases were in stage IA and the remaining case in stage IC. Follow-up information was available in 6 patients and the duration of follow up ranged from 1 to 10 years (mean=4.3 years). Five patients remained healthy, with no evidence of tumor recurrence. One patient died of tumor metastasis one year after the diagnosis. Gross examination showed that the tumor masses ranged from 7 to 20 cm in the greatest dimension (average=13.4 cm). Four of the 7 tumors were mixed solid-cystic in appearance and 2 cases were unilocular cystic in nature. Microscopic examination showed diffuse atypical follicular structures formed by granulosa cells. The granulosa cells contained round hyperchromatic nuclei, without nuclear grooves or Call-Exner body formation (6/7). In one of the cases studied, minor foci resembling adult granulosa cell tumor were also demonstrated. The degree of cellular atypia varied (3 cases with severe atypia, 1 case with moderate atypia and 3 cases with mild atypia). The mitotic count ranged from 1 to more than 5 per 10 high-power fields. Immunohistochemical study showed diffuse positivity for vimentin (6/6). The staining for cytokeratin (AE1/AE3) and calretinin was negative. Four cases expressed CD99 and 1 case was positive for inhibin. CONCLUSIONS Juvenile granulosa cell tumor is characterized by the presence of diffuse atypical follicular structures formed by small round cells, without nuclear grooves or Call-Exner bodies. Rare cases contain minor foci of adult granulosa cell tumor. They can be unilocular cystic in nature. The degree of nuclear atypia, mitotic activity and size of the tumor vary and do not correlate with the risk of recurrence and aggressive biologic behavior.