As part of the ISIS study, the prehospitalization phase in 570 patients with acute myocardial infarction was evaluated and protocolled in 34 Swiss hospitals. The aim was to assess whether the time lapse between onset of pain symptoms and start of treatment could be shortened. It was felt that, particularly in the case of lethal cardiac arrhythmias, rapid intervention could secure reversal or controlled relief of symptoms and/or conduction disorders. The study protocol recorded the specific time lapses between onset of symptoms and notification of the physician, notification of the physician and hospital admission, and between hospital admission and therapeutic action. The longest time lapse observed was the patient's delay (57%). Delay by the patient was shortened when pain symptoms (a) occurred during daytime, (b) were preceded by symptoms in the week before the infarction, (c) occurred at work, and (d) occurred during exercise, particularly in the company of unknown persons. The longest time lapse observed in hospital was associated with the night shift. The time lost through delay in contacting a doctor, the main factor in delaying therapeutic action in acute myocardial infarction, is very difficult to influence since the target group for a public information campaign is hard to identify. On the other hand, the "door to needle" time lapse observed in hospital can be reduced by the introduction of clear-cut guidelines.