Endostatin is an angiogenesis inhibitor that is an endogenously produced proteolytic fragment of type XVIII collagen. Serum levels of endostatin have been studied extensively in patients with malignant diseases. Recently, elevated serum endostatin levels were observed in patients with systemic sclerosis accompanying pulmonary fibrosis. To determine whether elevated serum endostatin can be observed in patients with idiopathic pulmonary fibrosis (IPF), we measured serum levels of endostatin in 69 patients with benign respiratory disease using an ELISA kit. The median of the serum endostatin levels in these patients was 50.8 pg/mL. Seven of 11 patients (63.6%) with collagen disease-associated pulmonary fibrosis (CDPF), and 19 of 24 patients (79.2%) with IPF had higher serum endostatin levels than the median level of the 69 patients. There was no statistical difference in serum endostatin levels between the patients with IPF and those with CDPF (P=0.7898). Serum endostatin levels in 24 patients with IPF were significantly higher than those in 34 patients with respiratory diseases other than IPF and CDPF (P=0.0001). Elevated serum levels of endostatin were observed in patients with IPF. Although the mechanisms are unclear, elevated serum levels of endostatin may be related to the fibrosing process in the lung.