A new clinical approach to patients with disorders of potassium excretion is reported. This approach uses a urinary index, the ratio of potassium concentrations in the urine to vein after adjusting the urine potassium concentration for medullary water abstraction. This index provides a semiquantitative assessment of the apparent transtubular potassium concentration gradient (TTKG) in the major distal nephron segment where potassium is secreted. Three clinical situations are presented where the use of this index provided a better indication of the renal action of mineralocorticoids than did the traditional approach; in each case, the presence of mineralocorticoids was known as drugs with this action were administered. We emphasize that use of this index is restricted to situations where the urine is not hypotonic and distal nephron sodium delivery is not limiting for potassium secretion (greater than 25 mM, twice the sodium concentration required for maximum potassium transport at this nephron site).