The purpose of this study was to assess the ability of [(18)F]FDG-PET, CT/MRI and serum tumor marker (TM) to predict the viability of residual masses after high-dose chemotherapy (HD-Ctx) in patients with metastatic germ cell tumors (GCT). In a prospective study, 60 residual tumors in 28 GCT patients were classified as viable/nonviable by FDG-PET, CT/MRI and TM levels. The results were validated either by histological examination of a resected mass and/or biopsy or by clinical/radiological follow-up for at least 6 months. There were no significant differences among the sensitivities observed with PET, CT/MRI and TM, but PET was significantly more specific than CT/MRI in predicting residual mass viability. TM showed the highest specificity. The highest accuracy in classification of residual tumors was achieved by a combination of PET, CT/MRI and TM (area under the ROC curve =0.91). All mature teratomas showed false-negative PET results with SUVs in the same range as necrosis. For classification of residual masses after HD-Ctx of metastatic GCT, [(18)F]FDG-PET is a valuable diagnostic method to complement the established procedures CT and TM. Positive PET results are highly correlated with the presence of viable tumor, but residual masses with negative PET findings still require resection. In cases of tumor progression diagnosed by CT and elevated TM, additional PET examinations are without benefit. PET seems useful in patients with stable disease or partial remission in CT/MRI and normalized TM as well as in marker-negative disease.