Hyperkalemia is often detected in young infants, particularly in association with acute pyelonephritis or a urinary tract anomaly. Cases of hyperkalemia in this population may also be due to transient pseudohypoaldosteronism, or immaturity of renal tubules in handling potassium excretion. Symptoms of hyperkalemia are non-specific, but are predominantly related to skeletal or cardiac muscle dysfunction, and can be fatal. Therefore, treatment has to be initiated immediately. Administration of fludrocortisone for hyperkalemia is appropriate in cases with hypoaldosteronism, but is challenging in young infants with hyperkalemia due to renal tubular immaturity, without pseudohypoaldosteronism. We report the case of a 25-day-old male presenting with persistent hyperkalemia with normal serum aldosterone, who was admitted with a first episode of pyelonephritis and unilateral high-grade vesicoureteral reflux. The patient was treated successfully with fludrocortisone.