Fifty-eight immature ovarian teratomas were studied. Neoplams with other germ cell elements (endodermal sinus tumor, choriocarcinoma, and dysgerminoma) were excluded so that the clinical and pathologic features of "pure" immature teratomas could be defined and correlated with the prognosis. The primary tumors and their metastatic growths were graded from 0 to 3. Forty were stage I; nine, stage II; and nine, stage III. The size and stage of teratomas were related to survival, but it was the grade of the primary tumor that best determined the likelihood of extraovarian spread, and it was the grade of the metastases that related best to the subsequent course. Actuarial survival was 63% at 5 years and also at 10 years. Regardless of the grade of the primary tumor, only one of six with grade 0 metastases progressed, and that neoplasms may not have been adequately sampled. Two of five neoplasms having grade 1 metastases did not progress, and two of six patients with grade 2 metastatic growths were living after relatively long intervals. All seven patients with grade 3 metastases died with tumor, none surviving more than 2.1 years. Survival of patients with grade 1, 2, and 3 neoplasms was 81, 60, and 30% respectively. The importance of adequate sampling of primary tumor and metastases for estimating prognosis and determining therapy is stressed.