Seven cases of ovarian "pure" immature teratoma were encountered in patients 10 to 38 years of age, six cases being in Stage Ia and one case in Stage IIc. The primary tumors and recurrent growth observed in one case were histologically graded from 0 to 3 according to the criteria of Norris et al. Karyotypes of the tumors and the patients were determined using culture and banding techniques. The only nonsurviving case was in Stage IIc. Four primary tumors belonging to grades 0, 1, and 2 showed a normal 46,XX female karyotype and the patients are alive and healthy. Three grade 3 tumors showed various types of karyotype abnormalities (48,XX,+14,+21; 47,XX,+20; 47,XXX). One patient died, one is alive after experiencing a recurrent tumor, and one has only been followed for 22 months. All seven patients had a normal 46,XX female chromosome constitution. Evidence to date indicates that karyotype of ovarian immature teratoma is either normal female 46,XX or a slight deviation from normal. It is postulated that in ovarian immature teratoma normal 46,XX karyotype is an indicator of favorable prognosis, whereas deviations in karyotype suggest a possibility of poor prognosis.