The levels of interleukin-8 (IL-8) in the serum, bronchoalveolar fluid (BALF) and epithelial lining fluid (ELF) were measured in patients with idiopathic pulmonary fibrosis. (IPF), in order to evaluate the clinical significance of IL-8. The serum levels were significantly higher in patients with active IPF (34.4 +/- 11.9 pg/ml, n = 8) than in those with stable IPF (mean: 14.6 +/- 10.9 pg/ml, n = 18), but neither correlated with the serum level of KL-6 or of SP-D, or with the intensity of chest Ga67-scintigraphy. There were no significant differences in BALF or ELF IL-8 levels between the active and stable IPF groups. These results suggest that the serum level of IL-8 is a useful marker for evaluating the disease activity in patients with IPF.