Although ICAM-3 is implicated in both adhesion and signal transduction events of leukocytes, its low affinity for LFA-1 compared to other ligands of LFA-1 has puzzled many investigators. Here we investigated the role of ICAM-3 in supporting LFA-1-mediated ICAM-1 binding and subsequently cell signaling. We observed that although ICAM-3 binds poorly to LFA-1 expressed on resting T cells, it specifically facilitates and increases LFA-1-mediated adhesion to the high affinity ligand of LFA-1, ICAM-1. We demonstrate that low-affinity binding of LFA-1 to ICAM-3 together with ICAM-1 alters the cell surface distribution of LFA-1 dramatically, inducing large clusters of LFA-1 that facilitate ICAM-1 binding after LFA-1 activation. We found that LFA-1-mediated ICAM-1 cell-cell interactions such as T cell proliferation greatly depend on low affinity LFA-1/ICAM-3 interactions that enhance stable LFA-1/ICAM-1 cell-cell contact. Taken together, these data demonstrate that low affinity LFA-1 binding to ICAM-3 regulates strong LFA-1/ICAM-1-mediated adhesion by driving LFA-1 into clusters to facilitate cell-cell interactions that take place in the immune system.