OBJECTIVE (i) To examine the usefulness of urinary net charge (UNa + UK - UCl) in the evaluation of hyperchloremic metabolic acidosis secondary to diarrhea, distal RTA and proximal RTA and (ii) To characterize the type of distal RTA on the basis of the underlying defect. METHODS Pediatrics division of a tertiary referral center. METHODS Thirty four children with hyperchloremic metabolic acidosis secondary to diarrhea (n = 16), distal RTA (n = 11) and proximal RTA (n = 7). Ten normal children with ammonium chloride induced acidosis were also studied. METHODS All subjects underwent urine collection of 30-60 minutes duration for measurement of Na, K, Cl, pH and pCO2. The measurements were also made on the blood samples collected at the midpoint of urine collection. The urinary net charge was calculated by subtracting Cl values from the sum of the Na and K. RESULTS Patients with proximal and distal RTA had a positive urine net charge. Patients with diarrhea and ammonium chloride induced acidosis showed negative urine net charge. Patients with diarrhea with extremely low urine sodium levels showed an inappropriately high urine pH despite persistent metabolic acidosis. All patients with distal RTA were found to have a secretory type of defect. CONCLUSIONS Measurement of urine net charge is helpful in the initial evaluation of a patient with hyperchloremic metabolic acidosis.