Introduction. Thrombocytosis is a disease where the platelet count exceeds 600,000/μl. It is usually reactive in nature, less often clonal. Reactive thrombocytosis is usually a response to reactive inflammation or infection, splenectomy, iron deficiency, pregnancy, physical effort or cancer. Case report. The case is described of a heart attack with ST-segment elevation in a 19-year-old male which was occurred due to intensive haematopoietic system renewal, caused by frequent blood donation which, in turn, caused reactive thrombocytosis. Reactive thrombocytosis is very rarely the cause of heart attack with ST-segment elevation, and is extremely rarely caused by blood donation. It is generally considered that reactive thrombocytosis is not a risk factor in the case of thromboembolic complications. Conclusion. The presented case and the literature point to the risk of serious thromboembolic incidents, including heart attack, in the course of thrombocytosis.