Abstract Ovarian cancer is mainly confined in peritoneal cavity and its metastasis is often associated with the formation of malignant ascites. As lysophosphatidic acid (LPA) is present at high levels in ascites of ovarian cancer patients and potently stimulates cell migration, we reason that LPA-stimulated cell migration may play an important role in ovarian cancer metastasis. Here, we show that only ovarian cancer cell lines with LPA migratory response undergo peritoneal metastatic colonization. LPA-stimulated cell migration is required for metastatic colonization because knockdown of LPA receptor subtype 1 (LPAR1) abolishes this event. However, the difference in metastatic potentials is not caused by the absence of LPAR1 because both metastatic and nonmetastatic lines express similar levels of LPAR1. Instead, we find that LPA can activate Rac only in metastatic cells and that metastatic colonization of ovarian cancer cells necessitates Rac activity. These results thus suggest that LPA-induced Rac activation is a prerequisite for ovarian cancer metastasis. In metastatic cells, Rac activation is facilitated by SOS1/EPS8/ABI1 tri-complex and the integrity of this tri-complex is essential for LPA-stimulated cell migration and metastatic colonization. We show that at least 1 member of SOS1/EPS8/ABI1 tri-complex is absent in nonmetastatic ovarian cancer cells and reexpressing the missing one conferred them with metastatic capability. Importantly, coexpression of SOS1, EPS8, and ABI1, but not of any individual member of SOS1/EPS8/ABI1 tri-complex, correlates with advanced stages and shorter survival of ovarian cancer patients. Our study implicates that the integrity of SOS1/EPS8/ABI1 tri-complex is a determinant of ovarian cancer metastasis. Cancer Res; 70(23); 9979–90. ©2010 AACR.