Elevated serum angiotensin converting enzyme (ACE) activity in sarcoidosis was usually reduced during steroid therapy but remained above the normal value (52 nmol .cntdot. min-1 .cntdot. ml-1) in approximately a third of patients during the 1st year of therapy. ACE was elevated in 5 of 10 patients treated with steroids for at least 4 yr. It was significantly higher in untreated patients (placebo group), who were subsequently placed on steroid therapy because of clinical deterioration, than in those remaining on placebo therapy. In general ACE diminished with increased disease duration. Serial study of ACE may be useful in clinical management of sarcoidosis, including activity assessment, need for steroid therapy, therapeutic result and prognosis.