To rule out renal acidification defects, 40 consecutive patients with their first renal stone episode were evaluated by measurement of fasting morning urine pH levels followed by a short ammonium chloride loading test in cases with high levels. Two patients (5%) exhibited incomplete distal renal tubular acidosis, the rest had normal urinary acidification. In view of the short duration of stone disease in the patients studied, the acidification defects were considered to be primary, and the stone formation secondary. The results justify extension of these simple screening procedures for unmasking renal acidification defects to include all patients with renal stone disease.