In 40 healthy persons, 11 nondialyzed uremic patients and 27 dialyzed uremic patients, of whom 4 also had cancer, the selenium concentration in serum was determined. The mean serum selenium concentration in healthy persons was 13.6 +/- 0.8 micrograms/dl, in nondialyzed patients 11.9 +/- 1.9 micrograms/dl and in dialyzed uremic patients 11.3 +/- 1.0 micrograms/dl. There was a significant statistical difference between healthy persons and uremic patients (p less than 0.001), but not between dialyzed and nondialyzed uremic patients. The subgroup of cancer patients on hemodialysis also had low serum selenium concentrations. The possible role of low selenium levels as one of the factors responsible for the increased incidence of malignancy in patients with chronic renal failure is supported by several facts such as: low serum selenium levels detected in patients with malignant diseases; the higher cancer incidence in inhabitants of low-selenium areas, and the known ability of selenium to inhibit many types of experimental carcinogenesis.