The magnesium retention test (MRT) was used together with muscle quadriceps lateralis biopsy in a pilot study of 5 patients with acute myocardial infarction (AMI) and 6 healthy control persons. Included were men aged 35-65 years with no prior medication and/or diseases. The magnesium retention was 42% in the infarction group and 22% in the control group (p less than 0.01). A lower muscle content of magnesium in the AMI group confirms the MRT and indicates a magnesium deficiency. The bleeding time after the magnesium load was prolonged.