Background: Macrophage inhibitory cytokine-1 (MIC-1/GDF15) mediates nonsteroidal anti-inflammatory drug (NSAID) protection from colonic polyps in mice and is linked to the development of colorectal carcinoma in humans. Therefore, changes in serum MIC-1/GDF15 levels could predict the presence of premalignant colonic polyposis and assist in population screening strategies. Methods: Serum MIC-1/GDF15 levels were measured in subjects in the Polyp Prevention Trial, in which NSAID use and colon cancer risk factors were defined. Subjects had an initial adenoma removed, a repeat colonoscopy removing previously unidentified polyps, and serum MIC-1/GDF15 estimation. Three years later recurrent adenomas were identified and serum MIC-1/GDF15 levels reestimated. The relationship between serum MIC-1/GDF15 levels and adenoma presence or recurrence was examined. Results: Serum MIC-1/GDF15 levels differed by adenoma status and were significantly related to colon cancer risk factors. In addition, mean serum MIC-1/GDF15 levels rose with increasing numbers of adenomas present and high-risk adenoma recurrence. NSAID users had higher serum MIC-1/GDF15 concentrations, which were related to protection from adenoma recurrence. Furthermore, adjusted serum MIC-1/GDF15 levels at final follow-up were related to adenoma recurrence (highest quartile MIC-1/GDF15; OR = 14.7, 95% CI: 3.0-73). Conclusions: These data suggest that MIC-1/GDF15 mediates at least some of the protection afforded by NSAIDs against human colonic polyposis. Furthermore, serum MIC-1/GDF15 levels vary with the development of adnenomatous colonic polyps. Impact: Serum MIC-1/GDF15 determination may hold promise as the first serum screening test to assist the detection of premalignant adenomatous colonic polyposis.