Of 109 teratomas in children 86 were benign and 23 malignant. Sacro-coccygeal and pelvic teratomas predominated 52 cases, and these fell into three groups, post-sacral, dumb bell and pre-sacral. The 34 purely posterior tumours were always congenital and benign whilst the incidence of malignancy in dum bell and pre-sacral teratomas increased as the tumour became more internal. Other sites affected in order of frequency were: the gonads, head and neck, retroperitoneal anterior mediastinum and the central nervous system. Malignant teratomas were carcinomas usually containing glandular, capillary and clear cell areas, and metastases were similar. Immature tissues in benign teratomas were usually neural or connective tissue. They did not give rise to neuroblastomas or sarcomas and did not indicate a worse prognosis. Only two originally benign teratomas later developed malignancy.