The presence of colonic tumor cells in the circulation may predict colorectal carcinoma recurrence and metastases. We have developed a highly sensitive nested RT-PCR assay, with primers derived from the cytokeratin 20 (CK20) and the carcinoembryonic gene CGM2, to detect occult microdisseminated enterocytes in blood of colorectal cancer patients. Among 82 healthy controls analyzed, 40.2% (33/82) have a positive expression of CK20 mRNA which is not statistically different from the 45.5% (15/33) of positive results found in colon cancer patients. This sensitive method may detect non-tissue specific constitutive low level (illegitimate) expression of CK20 mRNA in peripheral nucleated blood cells (PNBC) of a significant number of healthy control as well as in a number of normal bone marrow. The low specificity of this assay therefore hampers its value to detect blood colon cancer dissemination. In 47 patients with colorectal carcinoma, CGM2 primers detected circulating enterocytes in 25 of them (53%). In disseminated Dukes' stage C disease patients, 17 out of 29 (59%) were found positive whereas in localized adenocarcinoma (Dukes's stage A and B), CGM2 primers detected enterocytes in 44% suggesting that an hematogenous spillage of colonic cells may be a relatively early event in colon cancer. None of the patients suffering from benign colonic pathologies or from diverticulitis were found positive for this assay. The analysis of 56 healthy individuals without known colorectal cancer, of 20 non-colorectal cancer patients and of 6 normal bone marrows provide evidence that this assay is highly specific and may predict an hematogenous spread of colonic cells in patients with organ-confined disease. Nevertheless, the clinical significance of enterocyte detection and the potential applications of this molecular tool merit longer term follow-up.