['To investigate the association of left ventricular outflow tract (LVOT) obstruction with blood coagulation, platelet activity and inflammatory response in patients with hypertrophic cardiomyopathy (HCM) and sinus rhythm.', 'In 42 patients with HCM with sinus rhythm, including 16 patients with resting LVOT obstruction (gradient > or = 30 mm Hg) and 29 age- and sex-matched controls, markers of thrombin generation (thrombin-antithrombin complex (TAT), prothrombin fragment 1+2 (F1+2)), platelet activation (soluble CD40 ligand (sCD40L), beta-thromboglobulin (beta-TG), P-selectin) and inflammation (C-reactive protein (CRP), interleukin (IL)6, tumour necrosis factor-alpha (TNFalpha)) were determined.', 'Thrombin, platelet and inflammatory markers were higher in the entire HCM group than in controls (p<0.005 for all compared parameters). Compared with non-obstructive HCM, obstructive HCM was associated with increased thrombin formation (TAT, F1+2), platelet activation (sCD40L, beta-TG, P-selectin) and both CRP and IL6 levels. Only the level of TNFalpha was similar in both forms of HCM. In contrast, a comparison of non-obstructive HCM with controls showed that all these variables (except for P-selectin) were similar; P-selectin was higher in non-obstructive HCM. The LVOT gradient correlated positively with all the raised blood markers (r from 0.39 to 0.73; p<0.05), except for TNFalpha. In multiple regression analysis models, the LVOT gradient was the only independent predictor of TAT (R(2) = 0.61; p<0.001), sCD40L (R(2) = 0.59; p<0.001), F1+2 (R(2) = 0.55; p = 0.002), P-selectin (R(2) = 0.49; p = 0.004) and beta-TG (R(2) = 0.38; p = 0.005) in patients with HCM.'] Is obstructive hypertrophic cardiomyopathy associated with enhanced thrombin generation and platelet activation?