We evaluated whether recent cocaine use alters the specificity of CK-MB, myoglobin, and cardiac troponin I for acute myocardial infarction (AMI) in patients who are seen in the emergency department for chest pain. Patients <60 years old with potential myocardial ischemia underwent a standardized history and physical examination and routine CK-MB assays every 8 to 12 hours and had study serum obtained at presentation for CK-MB, myoglobin, and cardiac troponin I immunoassays, as well as benzoylecgonine, cocaine's main metabolite. We enrolled 97 patients, 19 (20%) of whom had recent used cocaine. Patients with and without cocaine use were similar with regards to sex, race, renal and muscular disease, diabetes, family history, and hypertension and rate of AMI (12% vs 11%, p = 1.0). In patients without MI, the mean myoglobin level was higher in cocaine users than noncocaine users (179 vs 74 ng/ml; Mann-Whitney p = 0.003), but the mean values were similar for CK-MB (2.2 vs 2.1 ng/ml; Mann-Whitney p = 0.58) and for cardiac troponin-I (0.02 vs 0.02 ng/ml; Mann-Whitney p = 0.87). The specificities of the markers in patients with and without cocaine use were as follows: cardiac troponin I, 94% vs 94%, (p = 1.0); CK-MB, 75% vs 88% (p = 0.24); and myoglobin, 50% vs 82%, (p = 0.02), respectively. Our data demonstrate that the specificity of myoglobin was altered by recent cocaine use. The specificity of CK-MB was affected less and the specificity of cardiac troponin I was not affected by recent cocaine use. (Am Heart J 1998;135:245-52.)