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The amount of fluid withdrawn by ultrafiltration in a dialysis session plays an important role in regulating the acid-base status of patients. It has been previously demonstrated that an interdialytic weight gain of 3 kilograms requires the removal of 3 liters, mostly of extracellular fluid, which may contain 60-70 mMols of bicarbonate. Such losses require an increase in the buffer mass transfer to achieve a good buffer balance. The importance of interdialytic weight gain (IWG) on acid-base status was evaluated in two significantly different periods. In the period where the IWG was lower, predialytic pH and HCO3 were significantly higher than in the alternate period. Since dialysis schedule, dialysate buffer, daily protein intake and given medications did not differ during the two periods, we conclude that a reduced ultrafiltration due to less weight gain betters predialytic acid-base status.