In 653 patients with suspected acute myocardial infarction the course of pain according to subjective assessment and morphine requirement is described. Patients were asked to score pain from 0-10 until a pain-free interval of 12 hours appeared. Different categories of patients constructed from clinical aspects were compared. Although the variability between groups was fairly small, subgroups were found in which the initial intensity of pain was more marked and the duration of pain was longer. Thus patients with larger infarcts according to maximum serum enzyme activity and patients with Q-wave infarction had more severe pain initially and also a longer duration and a higher morphine requirement compared with patients with a lower serum enzyme activity or a non-Q-wave infarction. Other groups with a more severe course of chest pain were those with more intensive pain at home, electrocardiographic signs of acute myocardial infarction on admission to hospital, and finally those with a high systolic blood pressure or a high rate-pressure product on admission to the Coronary Care Unit. We thus conclude that there is a variability of chest pain in suspected acute myocardial infarction and that there are defined groups of patients in which a more severe course of chest pain could be expected.