Previous studies showed that patients with neoplasms of various types and origins have abnormally high concentration of DNA in their serum. The current work compares circulating DNA levels in patients with benign or malignant disease of the gastrointestinal tract and determines the diagnostic value of such measurements. DNA was quantitated by radioimmunoassay capable of detecting 25 ng/ml, and as a simple and noninvasive test, it could be a useful addition to other diagnostic procedures. The GI tract was chosen because it affords a comparison of benign, precancerous, and malignant lesions of the same organ. Of the 386 patients studied prospectively, 48% had benign disease and mean DNA levels (±SE) of 118 ± 14 ng/ml, whereas 52% had malignant disease and 412 ± 63 ng DNA/ml. The difference was statistically significant (P < 0.001). The DNA assay showed the highest sensitivity for pancreas carcinoma: 90% of the patients had DNA levels above 100 ng/ml, chosen as the upper normal limit. Simultaneous measurements of both DNA and carcinoembryonic antigen (CEA) resulted in increased sensitivity and specificity, even when either marker alone had low sensitivity (gastric carcinoma). The results indicate that serum DNA concentration is markedly elevated in malignancy, and moderately elevated in benign disease, as compared with normal controls. These findings may have diagnostic and prognostic value.