Thirty-one patients with biopsy-confirmed sarcoidosis were studied for two to four years to compare serum angiotensin-converting enzyme (ACE) levels to clinical status, 67gallium scans, chest x-ray films, and pulmonary function tests (PFTs). Serum ACE levels and changes in ACE level correlated best with the clinical status of patients and their gallium scans (p less than 0.0005), and less with their chest x-ray films (p = 0.012) or PFTs (p = 0.007). The gallium scan was especially useful for localizing areas of disease involvement. Serial measurements of serum ACE were found to be a sensitive means for following the clinical course of patients with sarcoidosis and at times for predicting clinical relapse or improvement.