The dissemination of cancer cells is a prerequisite in the development of micrometastases and solid metastases. Our previous examinations of these cells were based on immunocytological staining of tumor-associated antigens and cytokeratins. We have now developed a highly sensitive and specific detection method based on a nested reverse-transcriptase-polymerase-chain reaction (RT-PCR) of cytokeratin-20 (CK-20) mRNA. Using this method, we examined the bone marrow of 57 patients with colorectal cancer and detected increasing numbers of CK-20-positive samples, depending on the UICC stage. Some 35% of all bone-marrow samples tested positive for CK-20: none were found in colorectal cancer stage 1, 24% were in stage II, 31% in stage III and 71% in stage IV. Investigation of bone-marrow specimens of patients with pancreatic cancer showed that 4 out of 11 patients were positive for CK-20 mRNA. To confirm that sample positivity for CK-20 expression was due to disseminated tumor cells, we examined bone marrow from a control group (n = 16) without apparent carcinoma. In this group, 15 out of 16 donors were CK-20-negative, while one donor with familial adenomatous polyposis showed a CK-20-specific signal.