Fifty-six measurements of angiotensin-converting enzyme (ACE) level, diffusing capacity (DLCO), and total lung capacity (TLC) were made in 18 sarcoid patients with a maximum of 25 months of follow-up observation. During spontaneous or corticosteroid-induced changes in disease activity, there was a significant inverse correlation between changes in ACE level and changes in DLCO and between changes in ACE level and changes in TLC. These observations suggest that ACE measurement may be a useful adjunct to pulmonary function tests to follow the course of sarcoidosis and to monitor the effects of corticosteroid therapy on the lung.