Serum angiotensin-converting enzyme (ACE) was studied in 51 patients with early or newly diagnosed sarcoidosis. Only 45% of these patients had increased ACE activity when their diagnosis was established, which diminishes the diagnostic value of this enzyme measurement. On the other hand, ACE accurately reflected disease activity, and it proved a useful tool for assessing of need for corticosteroid treatment. Patients with acute sarcoidosis associated with erythema nodosum (EN) had low ACE activity compared with the other patients with active, but less acute disease. Serum ACE was not significantly correlated with blood lymphocytes or the immunoglobulins, but there was a positive correlation between the enzyme and serum lysozyme, which strengthens the hypothesis of both enzymes being produced by the epithelioid cells of sarcoid granulomas.