The clinicopathologic features are presented of 8 malignant neoplasms occurring in adult teratomas, and of 2 malignant monodermal teratomas found in a continuous series of 1029 cases of ovarian adult teratomas. The age of the patients was 44--76 years. The first 8 observations included 6 epidermoid carcinomas, 1 undifferentiated carcinoma and 1 strumal carcinoid. In the epidermoid carcinomas malignancy was recognized peroperatively only in three instances. One tumor had infiltrated the broad ligament on one side and another the urinary bladder. The neoplasm appeared as a mass which thickened the wall of a typical dermoid cyst. Four patients died of the tumor from 3 months to 1 year after operation; one is alive more than 19 years later and one is lost to follow-up. The crucial factor in prognosis is the integrity of the ovarian capsule. The undifferentiated carcinoma, which had infiltrated the pelvic walls, could only be incompletely resected and caused death within 3 1/2 months. The strumal carcinoid was discovered by chance during laparotomy for a far-advanced adenocarcinoma of the pancreas which led to death within 2 months. The two cases of malignant monodermal teratoma comprised 1 thyroid papillary carcinoma and 1 carcinoid. Both patients are alive more than 8 and 5 years after operation. The carcinoid manifested itself by persistant diarrhea which ceased soon after operation and did not recur subsequently.