We evaluated the elimination of CO2 in three Japanese adults with carbonic anhydrase II (CA II) deficiency, as compared with that in 10 healthy volunteers. The patients had no signs of respiratory distress. Heart rate, body temperature, ventilation volume, respiratory rate and (a-ET) PCO2 were found to be higher and PaCO2 tended to be higher in the patients than those in the volunteers, while forced vital capacity (FVC), forced expiratory volume in 1s (FEV1), tidal volume, end-tidal PCO2 (PETCO2), pH and HCO3- in arterial blood were lower in the patients. All three patients had non-anion-gap metabolic acidosis, due to renal losses of bicarbonate, but with virtually no compensatory reduction in PaCO2. However, the high VE and PaO2 suggested that respiratory compensation for this metabolic acidosis was occurring, the elimination of CO2 was possibly facilitated by the presence of other carbonic anhydrase isoenzymes in the pulmonary capillaries. Thus, CA II deficiency causes mild to moderate impairments in CO2 elimination.