Cases of squamous cell carcinoma arising from mature cystic teratomas of the left ovary are presented. Two of the carcinomas occurred in postmenopausal women 54 and 71 years old and the third in a menstruating woman 42 years old. The two women with well-differentiated squamous cell carcinomas, who had extra ovarian extension of the tumor, died four and eight months postoperatively. The third woman with a poorly-differentiated squamous cell carcinoma, which was confined to the ovary, was alive two years after the operation. The special importance in the prognosis and spreading of tumors is emphasized.