Patients with systemic lupus erythematosus (SLE) have an increased risk of thrombosis. Platelet‐induced extracellular phosphorylation of plasma proteins suggests that this is due to persistent activation of the platelets. We examined 30 SLE patients (15 with thrombotic disease), 18 non‐SLE patients with deep vein thrombosis (DVT) and 50 healthy controls by analysing β‐thromboglobulin, activated factor XI‐antithrombin complexes and fibrinogen‐bound phosphate. All parameters were elevated in SLE patients, particularly those with thrombosis, but normal in DVT cases and healthy controls. We conclude that thrombotic disease in SLE patients is associated with a persistent systemic platelet activation that may lower the threshold for induction of thrombosis.