Serum levels of angiotensin-converting enzyme (ACE) were measured in 111 healthy subjects and in 117 adult patients with mediastinal and pulmonary sarcoidosis. The patients were followed up for 3 years. Detailed clinical investigations and ACE level measurements were performed every three months. The activity of the disease was assessed in 94 patients without treatment and in 23 patients under corticosteroid therapy. Mean ACE levels were significantly higher (p less than 0,0001) in all patients with sarcoidosis than in controls. They were also high (greater than 35 units/ml) in 32 patients with active forms of the disease. In 19 patients with inactive forms ACE levels were either moderately raised on normal, while considerable variations were observed in those with stable, persistent sarcoidosis. Complete spontaneous or therapeutically induced remission were accompanied by a return of ACE to normal values, but this was not the case for incomplete remission. There was no correlation between ACE levels and the radiological stages of the disease. Measuring ACE levels in patients with sarcoidosis may therefore contribute to diagnosis and surveillance and help to distinguish between complete and incomplete remissions.