We assessed the accuracy of C-reactive protein (CRP) levels and lymphocyte counts to predict a mechanical complication (MC) after myocardial infarction (MI). Within 10 years, we identified 36 patients with 39 echocardiographically confirmed MC within 30 days of MI: ventricular septal defect (17 cases), papillary muscle rupture (10 cases), and left ventricular free wall rupture (12 cases). They were compared to 41 controls with an uncomplicated hospital course after MI. Peak CRP levels and minimum relative lymphocyte counts obtained within 96 h of the acute MI (AMI) and before diagnosis of the complication were compared with clinical parameters. Prior to the MC, peak CRP levels were significantly higher (p < 0.001) and relative lymphocyte counts lower (p < 0.001) than in controls while creatine kinase levels did not differ (p = nonsignificant). Using multivariate logistic regression, the following score was identified to have excellent prognostic significance for MC: CRP (mg/l) - 10 x Lyc (%). The area under the receiver-operating characteristic curve was 0.90 +/- 0.05 (p < 0.001). Combined use of CRP levels and relative lymphocyte counts may be helpful in accurately predicting an MC after AMI and should therefore be routinely assessed.