Immature teratoma of the ovary is a rare tumour (1% of cancer of the ovary) usually seen in adolescents or young women. It is a malignant tumour derived from an abnormal germ-cell which undergoes meiotic division. The diagnosis is based on the pathological examination which reveals immature tissue derived from two or three types of embryonic tissue (endo-, meso- and ectoderm). Intraperitoneal dissemination occurs and immature or mature recurrence is observed. The prognosis was particularly severe before the use of polychemotherapy which has also made conservative surgery possible. At the present time, the consensus is to reduce the duration of the chemotherapy with a regular clinical and laboratory (tumour markers, aFP) monitory. A second look laparotomy verifies successful treatment.