Platelets are derived from the fragments that are formed from the cytoplasm of bone marrow megakaryocytes-small irregularly shaped anuclear cells. Platelets respond to vascular damage, contracts blood vessels, and attaches to the damaged region, thereby stopping bleeding, together with the action of blood coagulation factors. Platelet activation is known to affect genes associated with vascular risk factors, as well as with arteriosclerosis and myocardial infarction. Here, we performed a genome-wide association study with 352,228 single-nucleotide polymorphisms typed in 8,842 subjects of the Korea Association Resource (KARE) project and replicated the results in 7,861 subjects from an independent population. We identified genetic associations between platelet count and common variants nearby chromosome 4p16.1 ($p=1.46{ imes}10^{10}$, in the KIAA0232 gene), 6p21 ($p=1.36{ imes}10^{-7}$, in the BAK1 gene), and 12q24.12 ($p=1.11{ imes}10^{-15}$, in the SH2B3 gene). Our results illustrate the value of large-scale discovery and a focus for several novel research avenues.