Growing teratoma syndrome is defined as enlarging masses of mature teratoma following chemotherapy for malignant nonseminomatous germ-cell tumors. Typically, there is associated normalization of initially elevated serum tumor markers. We describe clinical and imaging findings in a case of growing teratoma syndrome originating from immature teratoma of the ovary in a 12-year-old girl. Familiarity with this unusual entity is important to avoid confusion with advancing malignancy.