In fetuses with sacrococcygeal teratomas (SCT), the development of hydrops is a harbinger of imminent fetal demise. In such instances, tumor debulking, either at fetal surgery or postnatally after urgent cesarean section, may prove lifesaving. Definitive SCT resection is carried out in the newborn period when the infant has stabilized. In five such patients, retrospective comparison of the SCT histological findings from the initial debulking procedure and later definitive resection or, in one case, autopsy results, showed a surprising change. After the initial debulking procedure, SCT histology findings from three patients were grade 3 immature teratoma without malignant elements, and in two patients grade 3 immature teratoma with nests of malignant yolk sac elements. In the three patients having grade 3 immature tumor without malignant elements, histological examination after definitive resection (two cases) and autopsy (one case) showed only fibrosis; there was no evidence of the SCT despite the fact that residual tumor had clearly been left at the initial debulking. In the patients in whom the initial histology finding was grade 3 immature with malignant elements, SCT histology results from the definitive resection showed grade 3 immature teratoma without malignancy in one case, and mature, benign tissue in the other. Preterm SCT debulking may induce tumor maturation or, alternatively, maturation of SCT that occurs naturally during gestation may be observed.