Zinc and copper levels in plasma have been measured in 41 patients with benign prostatic hypertrophy and 44 patients with carcinoma of the prostate, 24 of whom were receiving some form of hormonal therapy. The zinc concentrations in the blood leucocytes of some of the patients were also examined. We were unable to detect any differences between the leucocyte zinc levels of the three groups examined. Similarly, plasma zinc levels were not affected by age or disease, whereas the plasma copper levels were significantly higher (p less than 0.01) in the benign and malignant categories (mean = 124 micrograms/100 ml) when compared to a younger normal population (mean = 84.0 micrograms/100 ml). Hormonal therapy induced a rapid rise in the plasma copper concentrations and a concomitant marginal fall in zinc levels of the carcinoma patients. The magnitude of these hormone-induced changes were dependent on the type of therapy prescribed to the patient.