Ovarian germ cell tumor is a disease in which malignant (cancer) cells form in the germ (egg) cells of the ovary. Germ cell tumors begin in the reproductive cells (egg or sperm) of the body. Ovarian germ cell tumors usually occur in teenage girls or young women and most often affect just one ovary. The most common ovarian germ cell tumor is called dysgerminoma. See Ovarian Epithelial Cancer for signs, symptoms and treatments.