Survival in the acute phase of myocardial infarction and the subsequent prognosis are critically dependent on the time between onset of symptoms and medical intervention. Studies have shown that the time that patients take to decide to seek help accounts for most of the delay. We documented the length of time from onset of symptoms to arrival in hospital for 201 patients consecutively admitted to one of four hospitals in the Regional Municipality of Ottawa-Carleton between October 1986 and February 1987 for suspected acute myocardial infarction. Of the 160 survivors 42% waited more than 4 hours (a critical time for effective thrombolytic therapy) before coming to hospital, and nearly a third did not arrive within 6 hours. On the basis of interviews conducted with 42 patients, sociodemographic factors, education, past experience with an acute myocardial infarction, a previous diagnosis of angina and a coronary-prone behaviour pattern did not explain the delay. How patients perceived the seriousness of their symptoms and how they used other illness-related coping strategies explained 46% of the variance in the delay. Interventions aimed at reducing the delay between onset of symptoms and treatment must focus on patients' preadmission behaviour.