OBJECTIVE: To determine both the incidence and impact of infectious complications of acute myocardial infarction (AMI) on length of hospital stay and mortality. METHODS: This is a retrospective, case-control clinical trial involving medical records review. The study population consisted of patients admitted to the Coronary Care Unit (CCU) of the Hospital das Clínicas Heart Institute of the University of São Paulo Medical School – FMUSP – with AMI between January 1996 and December 1999. RESULTS: One thousand two hundred and twenty-seven patients were analyzed, and 60 (5%) met diagnostic criteria for infectious complication of AMI (infected group). The other 1167 patients served as control group. Mean age (67.5 versus 62.6), hospital length of stay (26.6 versus 12.0 days), and in-hospital mortality (45% versus 12%) were higher in the infected group. Mortality rate was higher among patients who underwent more than three invasive procedures (68% and 32%, p = 0.006). The most frequent infections were pulmonary (63%), urinary tract (37%) and positive blood cultures with no identifiable site of infection (8%). CONCLUSION: In the population studied, infectious complication rate was 5%. Prolonged hospital stay and high mortality rate suggest that infection complication has a great impact on AMI patients admitted to the coronary care unit.