Sarcoidosis is a systemic disease with predominantly pulmonary manifestations. Its frequency in Denmark is higher than previously assumed; on the basis of studies in two different areas the annual incidence was a least 10 cases/100,000 population. Because several cases remain undetected, the real incidence may be higher. The aetiology is unknown. Pathoanatomically the disease is characterized by the appearance of non-caseous epithelioid cell granulomas. The initial process in the lungs is presumably a non-granulomatous interstitial inflammation (alveolitis) with an accumulation of activated T-lymphocytes and mononuclear macrophages. Subsequent there is transition to organized granulomas and in some patients further development into fibrosis. The immunological abnormalities in peripheral blood suggest a stimulation of the humoral immunity and a inhibited cellular immunity. However, there is increasing evidence that the alveolitis may be an expression of increased cellular immunity manifesting at local sites of granuloma formation. ACE is a protein which in non-sarcoid individuals is associated with the endothelial cells, converting angiotensin I into angiotensin II and contributing to the bradykinin degradation. In sarcoidosis ACE is present in alveolar macrophages, epithelioid and giant cells. It can thus be considered as a marker for abnormal macrophage activity in the disease and has been introduced as a diagnostic tool. On examination of a widely compounded patient material we found elevated SACE in approx. 60% of sarcoidosis patients, compared with 1% in other conditions. Judged by these results, there was more than 90% probability that a patient with elevated SACE had sarcoidosis; however, a normal SACE did not preclude sarcoidosis. In newly detected sarcoidosis SACE was elevated in 50% of the patients, whereas elevated SACE was more frequent in patients with chronic active sarcoidosis (duration greater than 2 years). There was a large overlap between SACE values when the CXR stages were compared, a result which is comparable with other series. Two clinical manifestations exhibited peculiar enzyme patterns: in EN SACE was generally normal initially and subsequently increased to elevated values, and in hypercalcaemic sarcoidosis patients SACE was elevated in all. SACE was not elevated in EN of other aetiology or in non-sarcoid hypercalcaemia.(ABSTRACT TRUNCATED AT 400 WORDS)