Serum copper levels (SCl) were determined in normal patients and in patients with primary tumors of different sites, measurements being made before and after therapy. The serum copper was elevated in the presence of all the different types of tumor examined. In patients not responding to treatment and those who relapsed, the serum copper remained constant or became higher. SCl may provide an effective means of evaluating the extent of the disease and is of value in the estimation of prognosis after therapy.