Postmortem tissue analysis revealed significant potassium deficits in 58% of 27 patients maintained on chronic hemodialysis for periods of 2–75 months against dialysates containing 0–2.6 mEq/l of potassium. Potassium depletion was observed with all dialysate potassium concentrations but was greatest in patients dialyzed against potassium-free baths. The potassium deficit occurred primarily in skeletal muscle, myocardium, and brain. Myocardial potassium concentration was severely reduced in the majority of patients maintained on potassium-free dialysates and also correlated closely with muscle potassium concentration.