Mutated KRAS2 commonly can be detected in the plasma/serum of patients with pancreatic or colorectal cancers possessing this mutated gene. Positive assays are more common in patients with higher stage tumors but some smaller cancers can also be detected; occasionally, patients with large tumors have negative assays. Because relatively few patients with low-stage tumors have been evaluated, more studies in patients with smaller tumors are needed to further define the clinical usefulness of these assays. The reasons for variable results, particularly in patients with larger tumors, is unclear, although a variety of factors may be involved. More sensitive assays need to be developed that will increase the detection rates, although the problem of producing false positives must be minimized. The presence of mutated KRAS2 sequences in the plasma/serum seems to be quite specifically associated with the presence of cancer containing this mutated gene. This is an important feature of KRAS2 as a tumor marker. Preliminary studies in patients with pancreatic cancer suggest that assays for mutated KRAS2 can complement the commonly used CA19-9 assay and provide additional clinically useful information. The results from currently completed studies on the detection of mutated KRAS2 in patients with colorectal and pancreatic cancer are promising, and the potential usefulness of KRAS2 as a clinically important tumor marker should encourage future research.