The present invention is concerned with a protein oligomer comprising (i) at least two NC-monomers of collagen 18 or (ii) at least two endostatin domains of collagen 18 or (iii) at least two N-terminal peptides of the collagen 18 endostatin domain, for use in treating, ameliorating or preventing fibrosis or a fibrosis-associated disease, a vascular endothelial growth factor (VEGF)-related disease or a matrix metalloproteinase (MMP)-related disease. The invention further relates to the mentioned protein oligomer for use for detecting and/or diagnosing fibrosis or a fibrosis-associated disease, a vascular endothelial growth factor (VEGF)-related disease or a matrix metalloproteinase (MMP)-related disease.