Plasma magnesium concentrations were monitored daily in 86 patients who were admitted to a coronary care unit with a provisional diagnosis of acute myocardial infarction. Twenty-six patients had suffered a myocardial infarction, while the remainder had angina or non-cardiac chest pain. Magnesium levels were also obtained daily for five consecutive days in five normal subjects, who served as the control group. Calculations of the 95% confidence intervals on the differences between Day 1, Day 2 and individual subjects' mean plasma magnesium concentrations for the group with acute myocardial infarctions versus the group that did not have an acute myocardial infarction, the group with acute myocardial infarctions versus the reference group, and the group that did not have an acute myocardial infarction versus the reference group, and analysis of variance for the data from the three subject groups revealed no significant difference between the plasma magnesium levels of patients with an acute myocardial infarction and those with chest pain due to other causes or the normal reference population.