Induction of indoleamine 2,3-dioxygenase (IDO), an enzyme expressed by mononuclear phagocytes and some fibroblast cell lines in response to interferon-gamma, leads to enhanced degradation of tryptophan to kynurenine. Because inflammatory lung diseases are generally associated with activation of pulmonary macrophages, we investigated tryptophan metabolism in patients with interstitial lung disease by measuring circulating levels of tryptophan and kynurenine in peripheral blood and by measuring the IDO activity of bronchoalveolar cells. IDO activities were increased for bronchoalveolar lavage (BAL) cells obtained from patients with interstitial lung disease (115.4 +/- 30.4, n = 37) when compared with BAL cells from normal subjects (15.2 +/- 7.4, n = 14; p < 0.05), and messenger RNA for IDO was present in BAL cells from patients with interstitial disease but was not present in BAL cells from normal volunteer subjects. Patients with inflammatory lung disease also had decreased tryptophan and increased kynurenine concentrations in serum. The ratio of serum tryptophan levels to serum kynurenine levels was significantly depressed for patients with idiopathic pulmonary fibrosis (18.4 +/- 1.7, n = 29; p < 0.0001), patients with fibrosing alveolitis associated with collagen vascular disease (13.1 +/- 1.6, n = 18; p < 0.0001), or patients with sarcoidosis (21.0 +/- 1.1, n = 50; p < 0.0001), as compared with the ratio for normal subjects (31.8 +/- 2.3, n = 18). Patients with fibrotic disease had the highest levels of BAL cell IDO activity, and patients with collagen vascular disease associated fibrosing alveolitis had the most depressed levels of serum tryptophan and the greatest elevations in serum kynurenine. Measurement of tryptophan and kynurenine concentrations in serum may provide a useful measure of disease activity in chronic inflammatory parenchymal lung diseases such as sarcoidosis and idiopathic pulmonary fibrosis.