1. Preliminary studies are reported on 8 patients with lung disease given prednisone both with and without potassium supplements. 2. There were no abnormalities of plasma potassium but there was a relationship between the dose of prednisone and the urinary excretion of potassium whilst off supplements: higher doses were associated with increased potassium excretion. 3. Patients who had been on treatment for a short period retained more of their supplements than did those who had been on treatment for several years. 4. It is suggested that with prolonged treatment control of potassium homeostasis may be altered, and that more detailed metabolic studies should be carried out.