Platelet hyperactivity is important in the pathobiology of acute coronary syndromes. Glycoprotein V (GPV) is an integral membrane protein of platelets in the function of the GPIb-V-IX receptor for vWf/shear-dependent platelet adhesion in arteries. Soluble GPV is a novel marker of platelet activation. The aim of this study is to assess circulating soluble GPV levels in unstable angina pectoris (UA). Twenty-one patients ( 15 men, six women, aged 52 +/- 7 years) with UA pectoris were studied. The inclusion criteria were angina at rest lasting > 20 min during the preceding 6 h, with transient ST segment depression and/or T wave inversion and no evidence of myocardial infarction detected with the use of cardiac troponin-T. Coronary artery stenosis was angiographically confirmed in all patients. Twenty age- and sex-matched healthy adults ( 14 men, six women, aged 48 +/- 7 years) served as controls. There were no significant differences among the studied groups with respect to age, sex, obesity, smoking, total cholesterol, LDL-cholesterol, HDL-cholesterol, triglyceride and platelet counts. Plasma-soluble GPV concentrations were higher in the UA patient group ( 126 +/- 46 ng/ml) than those in the healthy controls ( 82 +/- 15 ng/ml) ( P = 0.001). There was a significant correlation only between plasma-soluble GPV levels and smoking (r = 0.526, P = 0.0001). Smoker UA patients had higher levels of soluble GPV than the non-smoker patients ( 139 +/- 40 vs. 113 +/- 50 ng/ml, respectively, P = 0.02). However, soluble GPV levels were similar in smoker and non-smoker healthy controls ( P = 0.2). It is concluded that soluble GPV concentrations are significantly increased during the acute clinical course of unstable angina pectoris, indicating that soluble GPV may be useful marker of platelet activation in those patients. The level of the molecule is significantly affected from smoking in those patients.