The homeostatic renewal of the intestinal epithelium depends on regulation of proliferation and differentiation of stem/ progenitor cells residing in a specific site, called the 'stem cell niche.' Thus, the reconstitution of the microenvironment of the stem cell niche may allow us to maintain intestinal stem/progenitor cells in culture for a longer period. Although epidermal growth factor (EGF) is conventionally used as a supplement of intestinal epithelial cell culture, little has been known regarding a role of EGF signaling in a stem/progenitor cell population. In this study, we attempted to clarify the role of EGF signaling in intestinal stem/progenitor cells, and to establish a culture system in which these cells could be maintained with normal differentiation potential. We first examined the expression pattern of EGF and its receptor, EGFR, and inhibited EGF signaling in mouse intestines. Next, we cultured intestinal cells isolated from mouse and human intestines in the presence of EGF and analyzed the function of EGF signaling in cultured cells. In both embryonic and adult mouse intestines, EGFR and EGF were expressed in immature epithelial cells and adjacent fibroblasts, respectively, and EGF signaling was essential to activate proliferation and inhibit apoptosis of intestinal stem/progenitor cells. Activation of EGF signaling also stimulated proliferation and suppressed apoptosis, both of which are necessary to maintain mouse and human intestinal epithelial cells in culture. Moreover, in these cultured epithelial cells, putative intestinal stem/progenitor cells persisted longer, and gave rise to different types of differentiated intestinal epithelial cells. We conclude that EGF signaling is indispensable for activation of proliferation and inhibition of unexpected cell death, not only in the intestinal stem cell niche, but also in culture of primitive intestinal epithelial cells.