Microalbuminuria, a subclinical rise in the urinary albumin excretion, is a risk indicator of atherosclerotic cardiovascular disease. The aim of this study was to measure the urinary albumin excretion in patients with acute myocardial infarction, and to correlate this with known atherosclerotic risk factors. One-hundred-and-twenty-six patients and 56 healthy controls matched for age and sex were studied. The albumin/creatinine concentration ratio in morning urine specimens was calculated as an index of the albumin excretion. Microalbuminuria was defined as a urinary albumin/creatinine concentration ratio above 1 mg/mmol. Urinary albumin excretion (0.88 [95% confidence interval 0.69-1.11] versus 0.51 [0.40-0.63] mg/mmol; p = 0.001) and frequency of microalbuminuria (33 [95% confidence interval 25-41] versus 16 [9-23]%; p = 0.03) were higher in patients than controls. This difference was independent of blood pressure, body weight, smoking, diabetes mellitus, renal disease, and thrombolytic treatment. There was a positive correlation between urinary albumin excretion and thickness of the left ventricle wall (R = 0.28; p = 0.001) which was independent of blood pressure. Follow-up examination of the patients will reveal whether microalbuminuria increases the risk for recurrence of acute myocardial infarction.