Cetuximab is a monoclonal antibody that binds to the EGFR, and is used in patients with colorectal cancer. The most common toxicities associated with the use of cetuximab are rash and hypomagnesemia. Hypomagnesemia is a major adverse event, but it has often been ignored in past studies and its management has not been characterized. The aim of this study was to obtain a better understanding of the overall incidence of hypomagnesemia and to evaluate the usefulness of our original treatment guidelines for hypomagnesemia in patients receiving cetuximab-based therapy. We investigated 15 patients who were treated with cetuximab(with or without combined chemotherapy)between October 2008 and November 2010. Thirteen patients developed hypomagnesemia: 11 patients had Grade 1, one patient had Grade 2, and one patient had severe hypomagnesemia(Grade 3). Grade 1 hypomagnesemia was observed after an average of 7. 5±4. 8 weeks of treatment. None of the patients developed Grade 2 or higher hypomagnesemia after the implementation of our treatment guidelines. In conclusion, cetuximab treatment is associated with a significant risk of hypomagnesemia. The early monitoring and effective management of hypomagnesemia are important for patients receiving cetuximab-based therapy.