Background: Thrombosis has been associated to some diseases like hyperadrenocorticism (HAC). Several drugs can alter the balance, such as the corticosteroid prednisone, used mainly for its anti-inflammatory and immunosuppressive effects. It is known that hypercortisolism can stimulate thrombi formation by increasing coagulation factors and decreasing fibrinolysis. However it is not known how prednisone administration affects hemostasis in dogs and if it is dose dependent. The aim of this study, therefore, was to demonstrate the effects of prednisone administration on dogs' hemostatic profile.Results: Significant decrease of antithrombin levels was observed in both groups (anti-inflammatory and immunosuppressive doses) after 15 days of treatment. An increase of platelet aggregation was observed in dogs receiving immunosuppressive doses of prednisone (Group II).Conclusions: From the results obtained in our study, it is not possible to infer that hypercortisolism can increase the thromboembolic risk, despite the decreased anticoagulant factors (antithrombin levels).Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)