The insulin-like growth factor I receptor (IGF-IR) is a ubiquitous and multifunctional tyrosine kinase that has been implicated in breast cancer development. In estrogen receptor (ER)-positive breast tumors, the levels of the IGF-IR and its substrate, insulin-receptor substrate 1 (IRS-1), are often elevated, and these characteristics have been linked with increased radioresistance and cancer recurrence. In vitro, activation of the IGF-IR/IRS-1 pathway in ER-positive cells improves growth and counteracts apoptosis induced by anticancer treatments. The function of the IGF-IR in hormone-independent breast cancer is not clear. ER-negative breast cancer cells often express low levels of the IGF-IR and fail to respond to IGF-I with mitogenesis. On the other hand, anti-IGF-IR strategies effectively reduced metastatic potential of different ER-negative cell lines, suggesting a role of this receptor in late stages of the disease. Here we examined IGF-IR signaling and function in ER-negative MDA-MB-231 breast cancer cells and their IGF-IR-overexpressing derivatives. We demonstrated that IGF-I acts as a chemoattractant for these cells. The extent of IGF-I-induced migration reflected IGF-IR levels and required the activation of phosphatidylinositol 3-kinase (PI-3K) and p38 kinases. The same pathways promoted IGF-I-dependent motility in ER-positive MCF-7 cells. In contrast with the positive effects on cell migration, IGF-I was unable to stimulate growth or improve survival in MDA-MB-231 cells, whereas it induced mitogenic and antiapoptotic effects in MCF-7 cells. Moreover, IGF-I partially restored growth in ER-positive cells treated with PI-3K and ERK1/ERK2 inhibitors, whereas it had no protective effects in ER-negative cells. The impaired IGF-I growth response of ER-negative cells was not caused by a low IGF-IR expression, defective IGF-IR tyrosine phosphorylation, or improper tyrosine phosphorylation of IRS-1. Also, the acute (15-min) IGF-I activation of PI-3 and Akt kinases was similar in ER-negative and ER-positive cells. However, a chronic (2-day) IGF-I exposure induced the PI-3K/Akt pathway only in MCF-7 cells. The reactivation of this pathway in ER-negative cells by overexpression of constitutively active Akt mutants was not sufficient to significantly improve proliferation or survival (with or without IGF-I), which indicated that other pathways are also required to support these functions. Our results suggest that in breast cancer cells, IGF-IR can control nonmitogenic processes regardless of the ER status, whereas IGF-IR growth-related functions may depend on ER expression.