We report two Japanese patients with dermatomyositis whose serum hyaluronate levels were correlated with their disease activities. The diagnosis of one patient was primary idiopathic dermatomyositis, and that of the second patient was dermatomyositis associated with neoplasia. The serum hyaluronate level of the former patient was decreased by systemic corticosteroid therapy, and that of the latter patient decreased following the surgical resection of mammary carcinoma and subsequent chemotherapy. The disease activity of each patient was greatly improved after these therapies. The longitudinal measurement of serum hyaluronate concentration may be useful to estimate the disease activity of patients with dermatomyositis whose initial serum levels of hyaluronate are elevated.