Up to a quarter of ovarian masses originate from germ cells, and many of these are mature cystic teratomas. The secondary development of malignancy is a rare but well-known phenomenon in patients with ovarian teratomas. Squamous-cell carcinoma accounts for 80% of secondary malignant transformations of ovarian teratomas. We aimed to do an up-to-date systematic review of this rare malignant transformation. 64 suitable studies provided information on 277 patients. Squamous-cell carcinoma in mature cystic teratoma was mainly found in women aged more than 50 years, with high concentrations of squamous-cell-carcinoma antigen and cancer antigen CA125, and with ovarian tumours more than 100 mm in size. Patients with FIGO stage Ia tumours had better survival than those with more advanced disease. Complete resection together with hysterectomy, bilateral salpingo-oophorectomy and lymphadenectomy for patients with advanced disease, followed by adjuvant chemotherapy with an alkylating drug was associated with higher survival, radiotherapy was not. We make proposals for investigation and treatment of this rare disorder.