The present invention is directed to a method for detecting colorectal adenoma and/or colorectal carcinoma comprising the steps: a) providing an isolated sample material which has been taken from an individual, b) determining the level of C3a or a derivative thereof in said isolated sample material, c) comparing the determined level of C3a or a derivative thereof with one or more reference values. The invention is further directed to a method for discriminating between colorectal adenoma and colorectal carcinoma as well as to a method for monitoring the course of colorectal adenoma and/or colorectal carcinoma and/or the treatment of colorectal adenoma and/or colorectal carcinoma. Moreover, the invention is directed to a test system and an array for use in these methods. Furthermore, the invention is directed to the use of C3a as a biomarker for a detection of colorectal adenoma and/or colorectal carcinoma in an individual. Further, the invention is directed to method for determining whether a compound is effective in the treatment colorectal adenoma and/or colorectal carcinoma.