Observations of morning increase in the incidence of myocardial infarction, sudden cardiac death, and myocardial ischemia demonstrate that the timing of these events is not wholly random. The most extensive data on the time of onset of the pain of myocardial infarction were published in 1976 by the World Health Organization in the report of the Myocardial Infarction Community Registers from 19 European centers. Data from over 8,900 patients clearly demonstrated a peak incidence of onset of pain from 8 to 11 a.m. Data from the Multicenter Investigation of the Limitation of Infarct Size (MILIS) Study revealed a significant circadian variation with an increased incidence of myocardial infarction in the period from 6 a.m to noon. The morning increase was detected despite the fact that data concerning the time of awakening and prior medication usage were lacking in this patient population. This finding has been confirmed by Willich in a study of over 1,700 patients with infarct onset determined by creatine kinase timing. The patients showed a four-fold increased morning onset of myocardial infarction relative to occurrence at night. When the relationship of infarction onset to time of awakening is clarified, and possible precipitants evaluated, increased information about triggering processes will be available. Such information will assist in the design of preventive therapy which can be evaluated in randomized trials.