In this study of 35 patients with a definite or possible myocardial infarction seen within the first 5 h of the onset of symptoms and who could clearly define the onset of symptoms, the serum myoglobin was raised in 20% of those in whom blood was sampled within 1 h of the acute event. In 11 of the 35 (31%) patients the serum myoglobin was normal in the first blood sample. Of the nine patients seen within the first 2 h of the onset of symptoms who had frequent samples taken, the first raised serum myoglobin was recorded from 45 min to 3 h 15 min after the onset of symptoms. The serum levels started to rise 15–20 min before levels greater than 85 ng/mL were reached. In the absence of further chest pain more than one peak in serum myoglobin was recorded in 58% of the patients with acute myocardial infarction. The serum myoglobin level returned to normal within 24 h of the onset of symptoms in 44% of patients and within 36 h in 67%.