Endodermal sinus tumor/Yolk sac tumor: • Endodermal sinus (yolk sac) tumour is third most common malignant germ cell tumour of ovary affecting mostly children or young women. (Age group 14–20 years.) • Tumour is highly malignant and is most rapidly growing tumour of whole body. • Histologically the pathognomic finding is Schiller - Duval bodies which is a single papilla lined by tumour cells with a central blood vessel. • Due to high rate of growth, tumour usually presents with acute abdomen. • Unilateral in 100% cases therefore biopsy of opposite ovary is contraindicated. • Almost all cases are associated with raised AFP and Alpha 1 antitrypsin. • It is the most deadly malignant ovarian germ cell tumor. • Treatment: Chemotherapy. • It has a high propensity for rapid growth, peritoneal spread and distant hematogenous dissemination to the lungs.