It was shown in the study that activation of free radical reactions took place in platelets of patients with hypertrophic cardiomyopathy. The degree of this activation depended on prevalent clinical syndrome (angina, arrhythmia, heart failure). Maximal malonic dialdehyde concentration and decreased activity of superoxide dismutase were observed in platelets and plasma of patients with overt heart failure. These patients had highest number of spontaneous platelet aggregates. Close correlation between number of platelet aggregates and concentration of a final product of lipid peroxidation malonic dialdehyde (r=0,924, p<0.001) was found in samples of blood from patients who had markers of unfavourable prognosis. The importance of studying lipid peroxidation in platelets parallel with investigation of their aggregability in hypertrophic cardiomyopathy is discussed.