Geophagia has been associated with life-threatening hyperkalemia in five patients with chronic renal failure. All five patients were black and had been born in the southeastern United States. Four had had frequent hyperkalemia requiring at least one hospitalization, and two had had hyperkalemia with serum potassium concentration as high as 9.8 mEq/liter, resulting in cardiac arrest in one and paralysis, disorientation, and cardiac arrythmia in the other. Since riverbed clay contains as much as 100 mEq of potassium in 100 gm of clay, much of which is exchangeable at acid pH, the mechanism of geophagia-induced hyperkalemia appears to be the absorption of potassium released from clay after ingestion. After discontinuing geophagia, no new hyperkalemic episodes occurred in these patients.