Deep vein thrombosis (DVT) is associated with coagulation abnormalities, but evidence of excess platelet activity is scant. Soluble P-selectin is a marker of platelet activity, with high levels being found in patients with thrombotic disease. We measured soluble P-selectin by enzyme-linked immunosorbent assay (ELISA) in plasma from 89 patients with objectively confirmed DVT and in 126 healthy age- and sex-matched control subjects, and found higher levels in the patients (P = 0.011). Taking the risk of DVT with a level of soluble P-selectin < 238 ng/ml to be 1, the relative risk of DVT with a soluble P-selectin level >238 ng/ml was 2.1 (95% CI 1. 2-3.6). These high levels may be a reflection of a generalized hypercoagulable state that, with factors such as the presence of persistent thrombin generation, could be responsible for excess platelet activation.