In germ cell tumors (GCT), a growing mature teratoma during or after chemotherapy with decreasing tumor markers is defined as ´growing teratoma syndrome` (GTS) according to its first describer Logothetis et al. in 1982. Due to the small number of available cases worldwide, not much is known about this continuously growing tumor and its pathogenesis. Especially in cases with extensive and surgical uncontrollable tumor mass, specific therapeutic options and biomarkers early indicating presence of GTS are still lacking. In this study, GTS was stratified into a slow (< 0.5 cm / month), medium (0.5 – 1.5 cm / month) and rapid (> 1.5 cm / month) group based on the tumor growth rate. We analyzed the secretome of 3 GTS samples of each subgroup and 3 teratomas. The secreted proteins were isolated from ex vivo cultivated tissues and analyzed by liquid-chromatography coupled to mass spectrometry (LS-MS).