 Given the difficulties in assessing diagnosis, severity and biological activity of GvHD by clinical means only, objective parameters for specific GvHD assessment are highly desirable. Criteria for appropriate GvHD biomarkers have recently been defined, thereby stating that suitable validated markers for monitoring of chronic GvHD are still lacking. CK18-F is the first marker that mirrors the pathogenetic endpoint of GvHD i.e. GvHD-induced apoptotic activity in critical epithelial organs bowel and liver. It represents a new class of GvHD markers which are complementary to the previously recognized immune activation parameters and might thereby be valuable for establishing serological signatures diagnostic for GvHD. This marker may allow distinguishing active GvHD from irreversible end organ damage and other clinical conditions commonly observed after transplant. 