The morphology of 20 ovarian tumors reported as solid teratomas is reviewed and correlated with their clinical behavior, in an attempt to select histologic features of prognostic significance. Utilizing a grading system quantitating immature tissue of embryonic origin, a tendency was shown for fully mature tumors to follow a benign course, and for those composed predominantly of immature tissues to prove fatal. This method, however, was prognostically unreliable in 16 tumors. An additional but far more important prognostic guide in this series was provided by extra-embryonic tissue of yolk sac origin, which was present in variable amounts and diverse histologic patterns; all teratomas including this component were fatal, frequently with evidence of extra-abdominal metastases. In certain circumstances, immature embryonic teratomas were associated with widespread peritoneal implants, causing death from intestinal obstruction and recurrent ascites. It is important when considering prognosis and therapy to realize the metastatic potential of extra-embryonic elements in teratomas, as distinct from the tendency of immature embryonic components to implant locally and mature into noninvasive deposits, which may be amenable to surgery.