A teratoma is a nonseminomatous germ cell tumor composed of well-differentiated derivations from at least 2 of the 3 germ layers. Mature cystic teratomas are known to have low metabolic activity and do not exhibit increased FDG uptake on PET. We report a case of mature cystic teratoma of the retroperitoneum showing increased FDG uptake in the solid portion. Pathologically, the solid portion was composed of abundant central nervous tissue. Increased FDG uptake in a teratoma may be not only due to an immature or malignant component but also to the presence of central nervous tissue.