The authors describe a juvenile granulosa cell tumour occurring in a 30-year-old woman. Clinically, the presence of ascites was associated with a pelvic mass. The tumour was localized strictly to the ovary, so conservative surgery was performed, proving sufficient to remove all the tumour tissue. Erroneous histological results revealed microscopic signs of an ovarian fibrothecoma. The patient, who presented with ascites, underwent laparotomy and excision was found to be impossible. A second histological examination showed microscopic signs of an ovarian juvenile granulosa cell tumour. Adjuvant chemotherapy was administered. After four months of treatment, however, the patient died. The purpose of this study is to assess the difficulty in using histological examinations to evaluate these tumours.