We examined, in age subsets, 2643 patients with acute myocardial infarction. Clinical features and 1 year morbidity and mortality were compared in 203 young patients (<45 years), 1671 patients 46 to 70 years old, and 769 elderly patients (>70 years). Ninety-two percent of young patients were men, and a family history of premature coronary artery disease was more common in young patients (41% compared with 28% of middle-aged and 12% of elderly patients). More young patients were currently smoking cigarettes (82% compared with 56% of middle-aged and 24% of elderly patients), and only 8% of young patients had never smoked. Previous myocardial infarction and history of angina pectoris or congestive heart failure were less common (p < .001) in the young patients than in middle-aged and elderly patients. In-hospital mortality was only 2.5% for young patients, compared with 9.0% in middle-aged and 21.4% in elderly patients (both p < .001). Postdischarge 1 year mortality was also strikingly low in young patients, at 2.6% compared with 10.3% in middle-aged and 24.4% in elderly patients. The incidence of reinfarction during the 1 year of follow-up was similar in all subsets. The statistical significance of 65 variables as predictors of 1 year mortality and reinfarction was tested and the following found to be significant (p < .05): hospital discharge on antiarrhythmic drugs, digoxin, or diuretics; history of previous myocardial infarction or congestive heart failure; chest x-ray findings of heart failure; low ejection fraction; and atrial fibrillation. Thus, young patients entering the hospital have an excellent 1 year prognosis, but those with prior infarction in whom there are selected abnormal findings at hospital discharge comprise a subgroup that may benefit from early aggressive management. Circulation 74, No. 4, 712-721, 1986. THERE IS A relative paucity of information concerning the clinical features, natural history, and prognosis in young patients with acute myocardial infarction. Despite the relatively low frequency of myocardial infarction in the young population,' the potential for death and long-term disability make this entity an important clinical problem. A number of studies have examined the epidemiologic features and the coronary arterial anatomy in young adults with evidence of coronary heart disease,'-" and angiographic studies have demonstrated less extensive coronary artery disease

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Circadian variation of the onset of acute myocardial infarction has been noted in many studies and may carry important pathophysiologic implications. However, only a few previous studies have attempted subgroup analyses. In 4,796 patients with documented acute myocardial infarction, the time of symptom onset was recorded. As in other studies, the peak of onset occurred in the morning from 6:01 AM to 12:00 noon, and 28% of the population (1.16 times the average percentage for the other time periods) experienced symptom onset in that period (p<0.001). There was a second, lower peak (25%) in the evening between 6:01 PM and 12:00 midnight, which was also observed in some previous studies. We sought to determine whether or not the presence of subgroups with specific clinical characteristics would exhibit different patterns and thereby contribute to these peaks in the overall population. In patients with a history of congestive heart failure (n=606) or with non-Q wave infarction (n=832), a pronounced peak (29%o) occurred only in the evening. Two nearly equal peaks were observed in patients older than 70 years of age (n = 1,422), smokers (n=2,057), diabetics (n =767), women (n = 1,213), and patients taking ,B-blocking drugs (n = 847). Finally, in patients with a previous myocardial infarction (n = 1,104), no peaks were observed. In a subgroup of patients (n = 1,084) free of the most important modifying factors, there was a single very pronounced late morning peak (32%, 1.39 times the average percentage for the other time periods, p<0.001) without evidence of a second evening peak. It is concluded that marked differences in diurnal patterns of myocardial infarction onset occur in subgroups of patients with modifying factors, particularly non-Q wave infarction, smoking, ,B-blocker use, diabetes, prior congestive heart failure, and prior myocardial infarction. The circadian pattern observed in a given total population reflects the contributions of these subgroups. (Circulation 1989;80:267-275) A circadian variation in the frequency of onset of acute myocardial infarction has been described in a number of studies during the past 25 years.1-8 Most show an increased onset in the morning with a peak incidence between 6:00 AM and 12:00 noon, although a secondary peak in the late evening has also been reported in some studies.1-3,5-7 A circadian variation in onset of other *All editorial decisions for this article, including selection of reviewers and the final decision, were made by a guest editor. This procedure applies to all manuscripts with authors from the