Three hundred fifty cases of immature teratoma of the ovary and 10 cases of ovarian dermoid cyst containing microscopic foci of immature tissue were analyzed to evaluate the relation between the two tumors. Twenty-six percent of the immature teratomas contained grossly visible dermoid cysts. 10% of them were associated with a dermoid cyst in the contralateral ovary, and 9 (2.6%) were preceded by resection of a dermoid cyst from the same ovary. Microscopic foci of slightly or moderately immature tissue were found in the walls of two of the previously excised cysts. Follow-up information available in nine of the 10 cases of dermoid cyst containing microscopic foci of immature tissue revealed no evidence of recurrence 11 months to 7 years post-operatively. Dermoid cysts that were followed by immature teratomas were characterized by high frequencies of multiplicity and rupture of the cysts. The findings suggest a relation between dermoid cysts and immature teratomas that warrants further investigation.