The concept of occult micrometastasis has been recognized for several decades. Recently, the balance of evidence favours the hypothesis that micrometastatic cells impacts on survival in spite of few clinical large-scale trials. Immunocytochemistry using anti-cytokeratin monoclonal antibodies is often used to detect micrometastatic cells. The strategy for detection of micrometastasis would help in the design of "tailor-made treatment". Furthermore, the genomic analyses of micrometastatic cells will contribute to the development of more effective strategies to treat cancer patients as well as the clarification of the carcinogenesis and the mechanism of clinical metastasis which is the leading cause of cancer-related death.