Malignant tumors of epithelial tissue are the most common form of cancer and are responsible for the majority of cancer-related deaths in Western industrialized countries. As a result of progress in surgical treatment of these tumors, lethality is linked increasingly to early metastasis, which is generally occult at the time of primary diagnosis. For this reason, the direct identification of minimal residual cancer is of particular importance. The studies described below demonstrate the utility of immunocytochemical and molecular analysis in the diagnosis and characterization of minimal residual cancer. These methods not only can identify this critical stage of tumor progression but also may facilitate the development of therapies to prevent manifest metastasis.