Cardiac markers are important in the diagnosis of acute myocardial infarction, especially in patients with an equivocal electrocardiogram pattern. Recently, Cardiac troponin I (cTnI) immunoassays have been used w diagnose myocardial infarction, because of this marker's specificity, sensitivity, and duration of elevated values. Immunoassays are the standard for analysis of cTnI and other cardiac markers. This report documents two cases in which cTnI values were discrepant with the clinical diagnosis. Retrospective analysis identified heterophilic antibodies as responsible for the discrepancies in one and possibly both cases. These cases expound the limitations of immunoassays and illustrate that cardiac markers must always be evaluated in concert with clinical data.