Background: Testicular germ cell tumors (TGCT) in clinical stage I (CSI) are tumors confined to the testis without evidence of metastasis. Around 50% of all TGCT patients present with elevated serum tumor markers (TM) such as alpha-feto protein (AFP), beta-humanochoriongonadotropin (ß-HCG) and lactate-dehydrogenase (LDH). After ablatio testis, TMs usually return to normal according to half-life kinetics, however, in clinical stage IS (CSIS) TMs remain elevated or increase after surgery. Follow-up data on CSIS is scarce and our study aims to assess clinical characteristics and oncologic outcomes in a large TGCT cohort. Methods: In this multicenter study we collected data from 5 tertiary referring hospitals in Germany, included patients with CSIS and evaluated TM levels, treatment and the primary outcome relapse-free survival. False CSIS was defined as documented CSIS but TMs that returned to normal after respective half-life kinetics. Differences between predefined groups (chemotherapy, TM, true/false CSIS) was analyzed with fisher’s exact and chi-square test. Results: Overall, 2616 patient data files were analyzed. Forty-three patients (1.6%) were documented as CSIS of which 27 (63%) were true and 16 (37%) false CSIS. Six (14%) had seminomas and 37 (86%) non-seminomas. In the true CSIS group AFP, ß-HCG, AFP plus ß-HCG and LDH were elevated in 13, 6, 3 and 2 cases. Four true CSIS patients received surveillance, 21 had 3x or 2x courses of BEP (bleomycin, etoposide and cisplatin) and 2 carboplatin. Within the false CSIS group, 2 patients were treated with surveillance, 10 received 3x BEP, one 3x PEI (cisplatin, etoposide and ifosfamid) and 3 had carboplatin. Chi-Square test revealed no difference between true or false CSIS classification in respect to application of chemotherapy (any chemotherapy, p = 0.83). Relapse-free survival after 5 and 10 years was 88.9% and 77.8%, respectively. Three patients in the true CSIS group relapsed (2 seminomas had carboplatin, 1 non-seminoma had surveillance). All relapses were treated with 3 courses of BEP with no documented death in the CSIS population. Conclusions: Overall, less than 2% of all TGCT were documented CSIS of which 37% were falsely classified. We report a high proportion of relapse-free survival in CSIS treated with surveillance or BEP with a high heterogeneity in treatment patterns. Correct classification of CSIS remains of critical importance to avoid toxicity for patients that could be safely treated with surveillance.