Although immature teratoma of the ovary is a rare disease, its pathologic grading, especially between low-grade (grade 1) and high-grade (grade 2 or 3) immature teratomas, is important for optimal therapy and prognosis. This grading, however, is currently solely dependent on quantitation of neuroepithelial components as judged by subjective assessments. As we have recently successfully studied the maturation of vascular smooth muscle cells (SMCs) in other organs using an h-caldesmon to α-smooth muscle actin (α-SMA) ratio, we decided to use this ratio to investigate a potential link between teratoma grade and SMC maturation, in combination with Ki-67 index. Sixteen immature teratomas along with 5 mature teratomas of the ovary were studied and stained with antibodies to CD31, α-SMA, h-caldesmon, and Ki-67. The number of vascular SMCs calculated using the α-SMA/CD31 ratio did not differ between teratoma grades (except between grade 0 and 3), whereas the number of mature vascular SMCs calculated using the h-caldesmon/CD31 ratio and maturation state calculated using the h-caldesmon/α-SMA ratio reduced significantly as teratoma grade progressed from 0 to 3. Furthermore, these parameters were significantly lower in high-grade than in low-grade immature teratomas (P<0.05). Ki-67 labeling index, regardless of germ cell layer, also significantly increased with teratoma grade (P<0.05). These results suggest that not only the number of neuroepithelial elements but also vascular immaturity and proliferating cell counts are biomarkers for ovarian teratoma grading. Thus, assessment of the maturity of vascular SMCs may serve as a valuable diagnostic tool for assessing teratoma maturity.