We have studied 35 patients to find the occurrence of hyperchloremic acidosis during the recovery phase of diabetic ketoacidosis. At admission the patients had typical normochloremic acidosis, with increased anion gap exactly balancing decreased serum bicarbonate. In contrast, in 18 patients with phenformin-induced lactic acidosis, the increase in anion gap at admission was much greater than the decrease in bicarbonate. The difference between lactic acidosis and ketoacidosis may be explained by a slower rate of excretion of lactate than of ketone anions. After the patients with ketoacidosis were treated, the acidosis became predominantly hyperchloremic with normal anion gap. Failure to normalize serum bicarbonate is attributed to excretion of ketone anions in the urine.