The suitability of CK20 mRNA expression as a marker for the detection of minimum residual disease in patients with cancer of epithelial origin was evaluated. A sensitive nested RT-PCR assay with multiple replicates was optimised to detect a minimum number of circulating tumor cells expressing cytokeratin 20 (CK20) mRNA. Using this optimal procedure, we examined CK20 mRNA expression in ten epithelial and seven leukemic cell lines, in eight bladder tumors, in peripheral blood samples from 18 tumor patients and from 29 healthy controls and in 8 bone marrow samples from healthy donors. CK20 mRNA was found in 13 of 18 (72%) blood samples from patients with cancer of epithelial origin and in all the epithelial tumor cells tested. However, CK20 mRNA was also detected in 21 of 29 (72%) bloods, in 8 of 8 bone marrow samples from healthy donors and in 4 of 7 leukemic cell lines. These results highlight a requirement for either determination of threshold levels of CK20 normal expression or the development of quantitative techniques to distinguish between a tumor-specific CK20 gene expression and a low level background transcription of this marker. These results would also advise caution in using CK20 as a tumor specific marker in clinical investigations.