Thromboembolic complications in adults with systemic lupus erythematosus (SLE) are described in literature. We intended to investigate the activity of natural anticoagulants, such as C protein. S protein and antithrombin III (AT III) in children with SLE to obtain data concerning activity of the disease and thrombotic complications. The study population consisted of 36 children with SLE and of 51 healthy children serving as a control group. The results showed a significant decrease of S protein activity, with particularly low levels in the group of patients with the presence of anticardiolipin antibodies (aCL), and decrease of C protein activity in patients with higher activity of the disease. They appear to prove the involvement of these inhibitors in inflammation and, on the other hand, the activation of intravascular coagulation, a risk factor of thrombosis.