Dermatomyositis (DM) is a systemic microvasculitis predominantly involving the capillaries. We investigated the expression of thrombomodulin (TM) and endothelial cell nitric oxide synthase (eNOS) in microvessels of DM patients. Twelve patients with acute or subacute onset of proximal muscle weakness and erythematous rash over their faces and shoulders were included in this study. Serum creatine phosphokinase was elevated in almost all patients. Electromyograph showed a myopathic pattern in all patients. Muscle biopsies were performed in all patients and 10 non-DM controls and studied with histological, enzyme histochemical and immunohistochemical staining. von Willebrand factor, TM and eNOS antibodies were used as the primary antibodies. Perifascicular degeneration and inflammatory cell infiltration in the perimysium were noted in almost all patients. Non-special esterase staining was markedly positive in capillary and microvascular endothelium. Marked reduction in TM and eNOS staining was noted in DM patients in perimysium microvessels and perifascicular area capillaries. Vascular lesions in DM were not only limited to capillaries. The low expression of TM and eNOS in microvessels suggests the anticoagulation and vasodilation functions of vascular endothelium is reduced. DM is an inflammatory vascular endothelial disease.