Myoglobin is a 17,800-d heme protein present in skeletal and cardiac muscle that is rapidly released into the blood circulation following muscle injury in a temporal pattern. In patients with acute myocardial infarction (AMI), the serum myoglobin level becomes abnormal in about 2 hours, peaks in about 6 to 9 hours, and becomes normal in 24 to 36 hours after the infarction. Measurement of myoglobin between 2 and 12 hours after the myocardial infarction has high clinical sensitivity and specificity. However, false-positive results could be encountered because of skeletal muscle damage and renal failure, whereas false-negative results could be seen in patients admitted 24 hours after infarction. False-positive results due to skeletal muscle injury may be eliminated by determining the myoglobin-to-carbonic anhydrase III ratio. Myoglobin can be a good adjunct to electrocardiography in improving the efficiency of early diagnosis of myocardial infarction. Monitoring the myoglobin levels can also help in evaluating the success of thrombolytic therapy. Use of myoglobin as an early biochemical marker for the diagnosis of AMI, its limitations, and the methods for quantitation are discussed.