Objective: to evaluate correlation between activation of protein C and thrombophlebitis. Method: cross-sectional and analytical study, was carried out on 50 patients with thrombophlebitis and 70 people in control at Cardiology Departement, Cho Ray hospital from June 2006 to March 2007.Results: Mean value (± SD) of activated protein C in patients with thrombophlebitis was: 80.88% ± 31, 41%.