In patients with a normal plasma anion gap type of metabolic acidosis, knowledge of the rate of ammonium excretion can provide valuable information to determine if there is a renal cause for the disorder. Unfortunately, few hospital biochemistry laboratories offer routine determination of the urine ammonium concentration. Data are presented that demonstrate a direct linear relationship between the urine anion gap (Na++K+- CI−) and the urine ammonium concentration. In a 24-hour urine collection, the relationship is urine ammonium equals - 0.8 (urine anion gap) + 82 (r = 0.97 p less than 0.01). The applications of this index of ammonium excretion are discussed.