In this study we evaluated the disease specificity of bronchoalveolar lavage fluid angiotensin-converting enzyme (BALF-ACE), its correlation with cellular constituents of bronchoalveolar lavage fluid (BALF), and for sarcoidosis, with other proposed markers of disease activity. Furthermore, the question of the clinical value of BALF-ACE determinations in in interstitial lung diseases or any of its subgroups was addressed. The study population consisted of 222 patients, 69 with biopsy proven sarcoidosis, 3 with hypersensitivity pneumonitis, 4 with acute histoplasmosis, 27 with idiopathic pulmonary fibrosis (IPF), 4 with rheumatoid arthritis-related interstitial fibrosis, 9 with pulmonary drug toxicity, 16 with pulmonary malignancies, 26 with other parenchymal lung disease entities, and 30 in whom the final diagnosis remained indeterminate. Elevated BALF-ACE concentrations were seen in all diagnostic categories. In sarcoidosis BALF-ACE levels correlated well with lavage lymphocyte counts (r = 0.49; p less than 0.0001), in contrast to IPF where they correlated well with lavage neutrophil counts (r = 0.51; p less than 0.007). The correlation of BALF-ACE and serum-ACE was significant. In sarcoidosis the mean BALF-ACE level was lower for patients with Stage-I chest roentgenographic patterns (0.664 U/L), compared to those with Stage II (1.112 U/L) and Stage III (1.083 U/L). It was concluded that elevated BALF-ACE levels are not specific for sarcoidosis. The correlations of BALF-ACE levels with different cellular constituents of BALF suggest a different cellular origin of BALF-ACE. In sarcoidosis BALF-ACE levels correlate well with other proposed markers of disease activity and seem to reflect pulmonary activity better than serum ACE.(ABSTRACT TRUNCATED AT 250 WORDS)