Postsurgical mediastinitis (PSM) remains a major cause of morbidity and mortality in patients undergoing cardiac surgery procedures. Although prompt diagnosis is crucial in these patients, neither clinical data nor imaging techniques have shown enough sensitivity or specificity for early diagnosis of PSM. The aim of the present study was to assess the validity of blood cultures as a diagnostic test for the early detection of PSM in patients who become febrile after cardiac surgery procedures. During a 4-year period (1999-2002), patients who developed fever (>37.8 degrees C) in the first 60 days after a cardiac surgery procedure were evaluated. Blood cultures were drawn from these patients. PSM was defined as deep infection involving retrosternal tissue and/or the sternal bone directly observed by the surgeon and confirmed microbiologically. Three criteria for positivity of blood cultures were applied: bacteremia, staphylococcal bacteremia, or Staphylococcus aureus bacteremia. For purposes of the analysis, a positive blood culture in patients with PSM was considered a true-positive test and a negative blood culture a false-negative test. Otherwise, in febrile patients without PSM in the postsurgery period, a positive blood culture was considered a false-positive test and a negative blood culture a true-negative test. Blood cultures were drawn from 266 febrile patients in the postsurgery period. PSM occurred in 38 patients (26 cases due to S. aureus, 8 to Staphylococcus epidermidis, 3 to gram-negative enteric bacteria, and one to Pseudomonas aeruginosa). Within the 60-day postsurgical period, blood culture as a diagnostic test was most accurate in patients with S. aureus bacteremia, providing 68% sensitivity, 98% specificity, a positive predictive value of 87%, and a negative predictive value of 95%. If the analysis was limited to the period during which patients are at maximum risk for PSM (day 7-20), the values in patients with S. aureus bacteremia were as follows: 73% sensitivity, 98% specificity, 90% positive predictive value, and 93% negative predictive value. Blood culture is an accurate test for the early diagnosis of PSM in febrile patients after cardiac surgery, particularly in institutions where S. aureus is prevalent in this context. A negative blood culture practically excludes PSM and, during the period of maximum risk for PSM, the presence of S. aureus bacteremia should compel early surgical management.