The lungs have an important role in the synthesis of angiotensin I converting enzyme (ACE). In BAL fluid and serum the ACE activity was determined in 18 patients with sarcoidosis (11 with high intensity and 7 with low intensity alveolitis), 14 patients with lung cancer and 16 with acute bronchitis. The activity of ACE was examined by a reagent set produced by Boehringer Mannheim Biochemica Test-Combination ACE cat. no. 789/011. The ACE activity in the high intensity alveolitis group of sarcoidosis patients was significantly increased in BAL fluid and serum in comparison to other observed patients. On the other hand, in patients with lung cancer the ACE activity was also increased in comparison to acute bronchitis and referred norms, especially in BAL fluid. This findings suggest a role of neoplastic process in ACE secretion in the airways. Very low correlation observed between ACE activity in serum and BAL fluid indicates a separate mechanism of secretion.