Serum copper level (SCL) was studied by the atomic absorption technique in 103 patients with non-Hodgkin's lymphoma. SCL was increased in 61% of patients at diagnosis or during active disease; values within normal range were found in 88% of patients in complete remission. The difference between mean SCL during active disease and in remission was highly significant, independently of stage and histologic type, so that: a) Within the same clinical stage high SCL at diagnosis was associated with poorer response to therapy in stage II and stage III (respectively P = 0.033 and P = 0.049), but not in stage IV, where the complete remissions were only 8 out of 42. A shorter 5-year survival was also shown in stages III and IV with high SCL at diagnosis (respectively P less than 0.025 and P less than 0.05), but not in stage II where the deaths were only 3 out of 24. b) Within histologic types, SCL is a useful prognostic index of response to therapy and survival, although a statistically significant difference was only reached for poorly differentiated lymphocytic lymphoma. We conclude that SCL may be a good parameter of disease activity and a useful index of response to therapy and survival in non-Hodgkin's lymphoma.