An increased mean serum copper level was found in 149 patients with lung cancer when compared with 19 healthy people and 23 patients with non-malignant lung diseases. The level seemed to reflect the stage of disease, with asymptomatic patients showing the lowest values, and patients with metastatic symptoms the highest. In spite of significant differences between the groups of subjects the scatter in the values was large. Hence serum copper determinations can be of only limited importance for differential diagnosis or in assessing the clinical stage of cancer. No differences in copper levels were found between the groups of patients with different histological types of lung cancer.