PTH and Hypomagnesemia: Since Mg(2+) is known to affect blood Ca(2+) levels through regulation of parathyroid hormone (PTH) secretion, we investigated the correlation between Ca(2+) and Mg(2+) concentration in blood.<h4>Methods</h4>Between April 2012 and October 2015, blood Mg(2+) and Ca(2+) concentrations (albumin corrected value) of 22 colon cancer patients undergoing treatment with either cetuximab or panitumumab at Toyooka Public Hospital were measured simultaneously.<h4>Results</h4>Hypomagnesemia (of all Grades) was reported in 13 of 22 patients.