In four out of 23 renal transplant recipients treated with cyclosporine A (CyA) and prednisone, hyperchloremic metabolic acidosis with high serum potassium was detected. The patients with metabolic acidosis had a normal anion gap and they all had a urine pH less than 6.0. Plasma renin activity (PRA) was in the low normal range and serum aldosterone in the normal range. Following reduction of the CyA dose, serum bicarbonate increased, serum chloride and serum potassium fell. None of these parameters, however, became normal. Because of morphological changes and sustained reduced renal function, two patients were switched from CyA to azathioprine plus prednisone. Within two weeks after switching, serum bicarbonate, serum chloride, and serum potassium normalized. These data suggest, that CyA induces dose dependent hyperchloremic metabolic acidosis with high serum potassium in renal transplant recipients.