The levels of copper, zinc, calcium, manganese and magnesium have been monitored in the sera of patients suffering from various types of cancer. Only serum copper appeared to be of any diagnostic significance, its levels being above the normal reported range in the breast cancer, leukaemia and Hodgkin's lymphoma patients. In the case of breast cancer, serum copper is progressively elevated according to the stage of the disease. Serum calcium levels were also significantly lower in patients with tumours of the breast, gastrointestinal tract and cervix. The results suggest that serum copper levels could be of prognostic significance in breast cancer patients receiving radiotherapy.