Background: Colorectal cancer remains the second most frequent cause of cancer deaths in both sexes in the United States. Both fecal and blood tests using tumor related antigens are increasingly being used to monitor treatments and in some cases aid in diagnosing colorectal cancers. However, higher sensitivity and specificity are needed before an acceptable tumor antigen blood test for colon cancer is clinically useful. Methods: Using double monoclonal ELISA analysis, a new colon cancer antigen, CA 11-19, (100 kDa glycoprotein) was measured in 670 colonoscopy confirmed patients including normal, benign GI diseases, polyps and colon cancer in a blinded IRB approved study. Results: Using a cutoff of below 6.4 units/ml as normal, CA 11-19 was positive (>6.4) in 132 of 139 of colorectal cancer cases. 100% of 63 cases of Stage I and II were positive. Sensitivity of CA 11-19 for diagnosis of colorectal cancer was 95%. Negative results were found in 94% of normals (175/187) and 80% of benign GI diseases (161/202). The specificity for the assay was 80.2% for all benign groups combined (426 negative assays for 531 benign patients). Conclusions: Ca 11-19 is a serologic tumor marker for colorectal cancer with a demonstrated sensitivity of 95% and a specificity of 80%. Diagnostically, a positive assay result increases the odds of finding colorectal cancer by a factor of five. The test appears to be highly sensitive for detection of early stage colorectal cancer. Additional prospective studies are needed to validate the use of CA 11-19 as an aid in the diagnosis of colorectal cancer.