Circulating tumour cells play a central role in the metastatic process, but little is known about the relationship between this cellular subpopulation and the development of secondary disease. This study was aimed at assessing the presence of colonic cells in peripheral blood of patients with colorectal cancer in different evolutionary stages, by means of reverse transcriptase polymerase chain reaction (RT-PCR) targeted to carcinoembryonic antigen (CEA) mRNA. In vitro sensitivity was established in a recovery experiment by preparing serial colorectal cancer cell dilutions. Thereafter, 95 colorectal cancer patients and a control group including healthy subjects (n=11), patients with other gastrointestinal neoplasms (n=11) or inflammatory bowel disease (n=9) were analysed. Specific cDNA primers for CEA transcripts were used to apply RT-PCR to peripheral blood samples. Tumour cells were detected down to five cells per 10 ml blood, thus indicating a sensitivity limit of approximately one tumour cell per 10(7) white blood cells. CEA mRNA expression was detected in 39 out of 95 colorectal cancer patients (41.1%), there being a significant correlation with the presence of distant metastases at inclusion. None of the healthy volunteers and only 1 of 11 patients (9.1%) with other gastrointestinal neoplasms had detectable CEA mRNA in peripheral blood. By contrast, CEA mRNA was detected in five of the nine patients (55.6%) with inflammatory bowel disease. These results confirm that it is feasible to amplify CEA mRNA in the peripheral blood, its presence being almost certainly derived from circulating malignant cells in colorectal cancer patients. However, CEA mRNA detectable in blood of patients with inflammatory bowel disease suggests the presence of circulating non-neoplastic colonic epithelial cells.