Beta-Thromboglobulin (beta-TG) is a platelet-specific protein that is released into plasma when platelets are activated in vivo. The involvement of platelets in tumor metastasis has been shown in animal studies. To study the role of platelets in tumor metastasis in humans, we measured plasma concentrations of beta-TG in 20 patients with newly diagnosed lung cancer and 11 healthy control subjects. The mean plasma concentration of beta-TG was significantly elevated in patients with advanced or metastatic lung cancer when compared with concentrations in healthy controls or patients with limited disease. This study suggests there is in vivo activation of platelets in patients with advanced or metastatic lung cancer.