Platelets represent an important link between inflammation and thrombosis and play an important role in all stages of atherosclerotic lesion formation. Increased platelet activity and their tendency to clot formation favour the incidence of thrombotic complications, such as unstable angina pectoris (UA), myocardial infarction (MI) and sudden cardiac death, in the course of coronary artery disease (CAD). Mean platelet volume (MPV) reflects the average size of platelets and, under normal circumstances, ranges between 7.5 fL to 10.5 fL. Platelet-large cell ratio (P-LCR) is defined as the percentage of platelets that exceed the normal value of platelet volume of 12 fL in the total platelet count. Platelet size has been shown to reflect platelet activity; therefore MPV and P-LCR are a simple and easy method of indirect assessment of platelet stimulation. In general population, higher MPV values are associated with increased risk of CAD. Higher MPV and P-LCR values are observed in CAD patients compared to patients without coronary atherosclerosis. In acute coronary syndromes (ACS) the MPV value is higher in patients with myocardial infarction than in patients with unstable CAD. In cases of stable CAD, elevated MPV correlates with the severity of coronary artery involvement and is a predictive factor of ACS. In patients with acute MI high MPV value has been reported to have impact on the no-reperfusion phenomenon following a percutaneous coronary intervention (PCI). Therefore, MPV and P-LCR indices, combined with other prognostic parameters, may be an important element of various scoring systems used in long-term prognosis in both stable CAD and ACS.