Serum magnesium concentrations were followed during the course of coronary infarction and were correlated to the event on the basis of the creatinine kinase activity E.C. no. 2.7.3.2. Those patients admitted to the hospital at a very early stage of the infarction showed initial high serum magnesium concentrations which fell during the event to subnormal values and then normalized later. In the case of severe coronary infarction (maximum CK activity greater than 700 U/1) the subsequent decrease in serum magnesium concentration was more pronounced than in subjects with lower CK activity. Patients suffering from angina pectoris had normal serum magnesium values which did not show any concentration change during a clinical follow-up. The previously postulated correlation between low serum magnesium and high risk for myocardial infarction may be questioned in the light of our observations.