Serum angiotensin-converting enzyme was measured in 185 children of different ages, in 32 normal adults, in 21 children with sarcoidosis, and in 36 children with other diseases. Children had higher serum ACE volumes than did adults; an intermediate level was absorbed in adolescents. Serum ACE values did not vary according to sex. Children with sarcoidosis had significantly elevated serum ACE levels that closely reflected the activity of the disease. Our results indicate that serum ACE is useful in confirming the diagnosis of sarcoidosis in childhood, and is of greater value as a sensitive indicator for following the course of the disease and the effectiveness of steroid therapy.