Background & Aims: Late diagnosis of colorectal carcinoma results in a significant reduction of average survival times. Yet despite screening programs, about 70% of tumors are detected at advanced stages (International Union Against Cancer stages III/IV). We explored whether detection of malignant disease would be possible through identification of tumor-specific protein biomarkers in serum samples. Methods: A discovery set of sera from patients with colorectal malignancy (n = 58) and healthy control individuals (n = 32) were screened for potential differences using surface-enhanced laser desorption/ionization time-of-flight mass spectrometry. Candidate proteins were identified and their expression levels were validated in independent sample sets using a specific immunoassay (enzyme-linked immunosorbent assay). Results: By using class comparison and custom-developed algorithms we identified several m/z values that were expressed differentially between the malignant samples and the healthy controls of the discovery set. Characterization of the most prominent m/z values revealed a member of the complement system, the stable form of C3a anaphylatoxin (ie, C3a-desArg). Based on a specific enzyme-linked immunosorbent assay, serum levels of complement C3a-desArg predicted the presence of colorectal malignancy in a blinded validation set (n = 59) with a sensitivity of 96.8% and a specificity of 96.2%. Increased serum levels were also detected in 86.1% of independently collected sera from patients with colorectal adenomas (n = 36), whereas only 5.6% were classified as normal. Conclusions: Complement C3a-desArg is present at significantly higher levels in serum from patients with colorectal adenomas (P < .0001) and carcinomas (P < .0001) than in healthy individuals. This suggests that quantification of C3a-desArg levels could ameliorate existing screening tests for colorectal cancer.