Occasional observations on this subject were seen until 1995 when Engell reported the large study on circulating cancer cells in the peripheral blood. Engell's report generated great enthusiasm in our country also. But, these cytologic method have a low sensitivity. Consequently, studies on the circulating tumor cells have been abandoned. However, since 1987, RT-PCR and other molecular biologic method were introduced for the identification of circulating cancer cells and micrometastases in bone marrow as well as in lymphnode. Serial analysis of a large number of patients by well-qualified laboratories have been performed. Several authors have shown a significant correlation between detection of cancer cells and relapse in some forms of malignancies. These research may be beneficial in the staging of cancer, in monitoring the effectiveness of cancer treatment, etc. Detection of cancer cells in the peripheral blood likely to have a profound impact on the practice of clinical oncology.