Ovarian sex cord stromal tumours include neoplasms that derive from granulosa cells, Seroli cells, theca cells, Leyding cells and fibroblasts of gonadal stromal origin. We present patient C., aged 15, was admitted with a 12- month history of periodic abdominal pain, abdominal enlargement and amenorrhea. Menarche from 14 years of age, previously normal menstrual cycle, sexually active since 14 years of age, which led the patient to believe she was pregnant. After preoperative preparation, surgery was performed with a left transrectal laparotomy. On revision, a massive tumor formation was found, originating from the left ovary without involving adjacent organs and anatomical structures. Radical resection of the tumor was performed with the fallopian tube. The results of histopathological examination established the presence of juvenile type granulosa tumor, provisional stage pT1a, high mitotic activity (mitotic rate 40 in 10 fields of view), moderate nuclear atypia and extensive foci of necrosis. The tumor infiltrated tubular tissue in which foci of necrosis and tumor foci of juvenile type granulosa cells were present. The patient was discharged in satisfactory condition and referred to an oncology institution to determine further treatment tactics