OBJECTIVE--To determine the long-term prognosis of patients after a myocardial infarction (MI) at a young age. DESIGN--Prospective cohort study of patients aged 55 years or less suffering a myocardial infarction. SETTING--A single coronary care unit admitting patients from the community. PATIENTS--255 consecutive patients (210 men) aged 55 years or less admitted between 1981 and 1985 after acute MI. Twenty four patients died in hospital or within 3 months of infarction and 11 were lost to further follow up after discharge. Of the remaining patients, 150 (mean (SD) age 48 (5.7) years) able to exercise 3 weeks after infarction and who agreed to undergo coronary angiography were recruited to a study group and seen 18 months, and 3, 5, and 7 years after MI. In addition, a cross sectional analysis of survival was made to a median of 120 months. Seventy 3 month survivors (mean (SD) age 48 (5.8) years) were not recruited to the study group but were traced for late survival through their general practitioners and family health service associations to a median of 130 months. MAIN OUTCOME MEASURES--Survival in young patients after MI and the survival of 3 month survivors stratified by their ability to exercise and agreement to undergo angiography. The rate of coronary artery surgery (CAGB) and reinfarction during the first 7 years after index MI in patients recruited to the study group. RESULTS--Sixteen patients (6%) died in hospital and eight (3%) within 3 months of the index infarction. The 7 and 11 year survival rates in the whole cohort of 255 patients were 80% and 66% respectively using life table methods. Survival 7 years after MI, in patients recruited to the study group was better than in those not recruited (93% v 79%, P = 0.001), but thereafter mortality in the study group accelerated and there was no significant difference in survival 11 years after infarction (76% v 67%, P = 0.05). There was a trend towards higher mortality in patients with multivessel disease and severely impaired left ventricular function. During the first 7 years after MI, 38 of 150 patients in the study group underwent CABG and 19 suffered reinfarction, which was fatal in three. CONCLUSION--The medium-term prognosis of young survivors of MI is good, particularly in patients recruited to the study group. After 7 years there is an increase in mortality and the long-term prognosis is less favourable. This should be taken into account when planning future management and follow up of young patients after MI.