In a variety of human malignancies, tumorigenesis and the development of metastatic disease are accompanied by changes in the expression of cell adhesion molecules. In carcinomas, normally expressed cell adhesion molecules are lost, or expressed in a functionally altered form, events which may help tumor cells to escape from contact-mediated controls and leave the primary tumor. The development of metastatic potential is, in a number of solid tumors, associated with the expression of new cell adhesion molecules by the tumor cells. These newly expressed cell adhesion molecules appear to mediate tumor cell interaction with leukocytes and endothelium, and may direct dissemination of the tumor cells throughout the body.