Activation of focal adhesion kinase (FAK), overexpressed in several human cancers, induces survival, proliferation and motility of cells in culture, but its functional importance in human tumor growth in vivo has not been elucidated. I explored the role of FAK in regulating tumorigenicity of human carcinoma cells, HEp3. These cells overexpress urokinase receptor (uPAR) which, by activating alpha5beta1 integrin, initiates an intracellular signal through FAK and Src leading to ERK activation and tumorigenicity in vivo. Down regulation of uPAR in these cells led to an approximately 3-5-fold reduction in FAK phosphorylation and association with Src and dormancy in vivo. Both FAK phosphorylation and ability to grow in vivo were restored by re-expression of uPAR. The FAK signaling pathway in T-HEp3 cells, measured by FAK phosphorylation, GTP-loaded Ras and ERK activation, was inhibited by transient or stable transfection of FAK related non-kinase (FRNK), known to have a dominant negative function, but not by a FRNK mutant version (S1034-FRNK). Most importantly, while vector- and mutant-S1034-FRNK transfected cells inoculated onto chicken embryo CAMs formed progressively growing tumors, the HA-FRNK-expressing T-HEp3 cells did not proliferate in vivo and remained dormant for at least 6 weeks. Both cell types had similar rate of apoptosis in vivo and the p38(SAPK) or PI3K-Akt signaling pathways were unaffected by FRNK. FRNK induced dormancy could be reverted by expression of an active-R4F-Mek1 mutant. These results show that active FAK is an important mediator of uPAR-regulated tumorigenicity of HEp3 cells and that interruption of FAK mitogenic signaling either through down-regulation of uPAR or by expression of FRNK can force human carcinoma cells into dormancy.