Acquired protein S deficiency and lupus anticoagulant have been described in children with varicella who had pul-pura fulminans. disseminated intravascular coagulation or thrombosis. The sim of this study was to investigate the natural anticoagulants, hypercoagulability markers, other parameters of coagulation and fibrinolytic systems, and the presence of the lupus anticoagulant in otherwise healthy children with acute varicella infection. Blood samples were obtained from 17 children with varicella without thrombosis during acute varicella infection and 1 mo after onset. Coagulation tests included determinations of the prothrombin time, the activated partial thromboplastin time. the thrombin time, the thrombin antithrombin complex, the prothrombin fragment F 1 + 2, the tissue plasminogen activator, the plasminogen activator inhibitor-1, protein C activity and free protein S antigen. Antiphospholipid antibodies were determined in enzyme-linked immunosorbent assays. The mean free protein S concentration in the acute phase (0.63 +/- 0.16 U/ml) was significantly lower than that of the concentration determined 1 mo later (0.82 +/- 0.17 U/ml). The children with acquired free protein S deficiency also had a lupus anticoagulant. Elevated concentrations of the prothrombin fragment F 1 + 2, the thrombin antithrombin complex, D-Dimer, tissue plasminogen activator and plasminogen activator inhibitor-1 were detected in most of the children.