Detachment from the extracellular matrix causes apoptosis of normal epithelial cells--a phenomenon called anoikis. K-ras oncogene, an established anoikis inhibitor, often occurs in colorectal carcinoma (CRC). In addition to blocking anoikis-inducing mechanisms, oncogenic K-ras can cause anoikis-unrelated changes in CRC cells, such as induction of events promoting their deregulated mitogenesis, ability to trigger angiogenesis, and so on. Thus, whether ras-induced anoikis resistance of CRC cells is essential for their ability to form tumors in vivo or represents a mere epiphenomenon is unclear. We found that when poorly tumorigenic, oncogenic, K-ras-negative, anoikis-susceptible human CRC cells were cultured under anoikis-inducing conditions in vitro, they spontaneously gave rise to an anoikis-resistant cell population harboring de novo oncogenic K-ras mutations and manifesting dramatically increased tumorigenicity. We further observed that a variant of the same oncogenic K-ras-negative anoikis-susceptible cells selected for increased tumorigenicity acquired de novo oncogenic K-ras mutations and manifested increased anoikis resistance. Unlike the case with anoikis, oncogenic K-ras did not rescue CRC cells from death caused by hypoxia or anticancer agents. Taken collectively, our results support the notion that ras-induced anoikis resistance of CRC cells is essential for their ability to form tumors in vivo and thus represents a potential therapeutic target.