Background: Patients withmetastatic germ cell tumors of the testis (GCTs) are risk-stratified according to the International Germ Cell Cancer Collaborative Group (IGCCCG) classification system for further therapy decision. This risk classification is based on metastatic load as well as tumor marker levels of AFP, HCG and LDH assessed pre-chemotherapy after orchiectomy treatment. An incorrect classification is possible when pre-orchiectomy marker levels are used, possibly resulting in over- or undertreatment of patients. Therefore, the aim was to investigate the potential frequency and clinical relevance of incorrect risk stratification using pre-orchiectomy tumor marker levels. Methods: Patients with metastasized nonseminomatous GCT (NSGCT)were included in a multicenter registry analysis which was conducted by investigators of the German Testicular Cancer Study Group (GTCSG). Based on the marker levels at different timepoints, IGCCCG risk groups were calculated. The agreement was tested using Cohen’s kappa. Results: A total of698 of 1983 (35%) patients were diagnosed with metastatic NSGCTs, and 546 (78%) had sufficient data for 232 follow-up data points. By using pre-orchiectomy tumor marker levels, 111 patients (20%) would have been incorrectly classified. Seventy-five patients (14%) were classified into a higher risk category, and 36 patients (6%) were classified into a lower risk category. Cohen’s kappa was 0.69 (p < 0.001), showing a strong agreement between the use of both marker timepoints. The treatment of misclassified patients would have resulted in an overtreatment of 75 patients or undertreatment of 36 patients. Conclusions: The use of pre-orchiectomy tumor marker levels may lead to an incorrect risk classification and might subsequently lead to under- or overtreatment of patients. Part of these data were published in: Majewski et al. Cancer Med. 2023.