Patients receiving long-term total parenteral nutrition (TPN) are at risk for selenium deficiency. The purpose of this study was to determine the effect of parenteral selenium as selenious acid on the selenium status of seven long-term TPN patients. Patients received a dosage of zero, 80, or 160 micrograms Se/day for 1 month each. The measures of selenium status used were selenium levels in plasma and glutathione-peroxidase activities in erythrocytes and platelets. Urinary selenium excretion was measured. Control subjects were selected to match the sex, age, and weight of the patients. With increasing levels of parenteral selenium, there was increasing plasma selenium concentration as well as erythrocyte and platelet glutathione-peroxidase activity. There was no statistical difference between the patients during the time they received the 160 micrograms parenteral selenium treatment and the control subjects for platelet glutathione-peroxidase activity. At the 160 micrograms Se/day level, patient plasma selenium concentrations increased from 28% to 58% of the control levels. Four patients were studied after they returned to the 80 micrograms parenteral selenium/day from the 160-micrograms Se/day treatment. With decreasing parenteral selenium, three patients had decreasing platelet glutathione-peroxidase activity, while plasma selenium concentration decreased in two patients. These data suggest that some patients receiving long-term parenteral nutrition should receive parenteral selenium.