We present the case of 27-year-old male, physical training teacher in whom the first clinical sign of idiopathic thrombocytosis was myocardial infarction. The infarct was complicated by heart arrest caused by ventricular fibrillation. Streptolysis (streptokinase) was used in treatment. Thrombocytosis 842 G/l was observed in blood tests at admission. To estimate its cause the bone marrow biopsy and trepanobiopsy of hip bone were performed. Tests were performed to exclude the secondary cause of thrombocytosis. The coronarography did not show relevant stenoses of coronary arteries. The results let diagnose myocardial infarction in a patient with essential thrombocytosis.