A compromised mitotic checkpoint, the primary mechanism for ensuring that each new cell receives one copy of every chromosome, has been implicated as a contributor to carcinogenesis. However, a checkpoint response is shown here to be essential for cell survival, including that of chromosomally instable colorectal cancer cells. Reducing the levels of the checkpoint proteins BubR1 or Mad2 in human cancer cells or inhibiting BubR1 kinase activity provokes apoptotic cell death within six divisions except when cytokinesis is also inhibited. Thus, suppression of mitotic checkpoint signaling is invariably lethal as the consequence of massive chromosome loss, findings that have implications for inhibiting proliferation of tumor cells.