The study involving 48 patients with the preliminary diagnosis of acute coronary syndrome aged 54-78 years included analysis of clinical, laboratory and instrumental data for subgroups with different definitive diagnosis (myocardial infarction and unstable angina in 28 and 22 patients respectively). On the whole, the mean content of HSP-70 at admission averaged 2.1 ± 0.3 ng/ml and decreased to 1.6 ± 0.4 ng/ml (p < 0.05) after therapy. In the subgroup with myocardial infarction, the HSP-70 level (1.6 ± 0.4 ng/ml) was significantly lower than in patients with unstable angina (2.1 ± 0.3 ng/ml); p < 0.05. The reduced HSP-70 level associated with a severe myocardial lesion gives evidence against the hypothesis of these proteins as markers of myocardial dysfunction whereas a rise in their content at the onset of ischemic process suggests their protective role.