Serum K concentration is usually normal or moderately elevated in chronic renal insufficiency. Severe hyperkalemia, disproportionate to the degree of renal deficiency, has been frequently observed in chronic pyelonephritis; its pathogenesis is still unknown. Marked elevation of serum potassium was observed in a patient with chronic pyelonephritis and simultaneous selective deficiency of aldosterone. The hyperkalemia was corrected by mineralocorticoid replacement therapy. In some cases of chronic pyelonephritis selective aldosterone deficiency may be responsible for the marked hyperkalemia.