A study of 353 serum copper levels (SCL) in 236 patients with non‐Hodg‐kin's lymphoma has demonstrated, in general, a clinically useful relationship between SCL and disease activity. Elevated SCL in the active disease decreases as disease activity is reduced in response to therapy, with normal SCL in inactive phases and increased SCL in relapse generally preceding clinical signs. This relationship of SCL to disease activity is noted in all histologic groups and types of malignant lymphoma, with SCL values slightly higher in undifferentiated lymphoma. An analysis of apparently inconsistent SCL has been included. The relationship between SCL and extent of disease is shown by marked elevation of SCL in generalized disease as contrasted to localized disease. A detailed study of sequential SCL in 18 patients undergoing therapy also confirmed its clinical usefulness in the evaluation of status of disease activity and efficacy of therapy.