Levels of serum copper in 34 patients with adult non-Hodgkin's lymphoma at different phases of the disease have been studied. All of the patients were evaluated with complete blood counts, sedimentation rate, gallium scintigraphy, liver and bone marrow biopsies, lymph node biopsy, and laparoscopy. The level of serum copper was significantly elevated in non-responding or relapsing patients (mean 191.06 micrograms/dl), and correlated with the estimated tumor burden. Serum copper levels within normal range were found in patients in complete remission (mean 114.76 micrograms/dl). Age- and sex-matched normal controls also showed serum copper levels within normal range (mean 112.81 micrograms/dl). It is proposed that serial measurements of serum copper level may be of use in: (1) monitoring the remission status of patients with non-Hodgkin's lymphoma, (2) detecting early relapse of non-Hodgkin's lymphoma, and (3) contrary to previous reports by Hrgovcic et al., the level of serum copper seems to be related to the disease activity of histiocytic lymphoma.