Muscular dystrophy patients fall in respiratory failure in the terminal stage. Erythrocyte 2,3-diphosphoglycerate (2,3-DPG) is an important regulator of oxygen release, as it affects the position of the oxyhemoglobin dissociation curve. In order to survey the internal respiration in these patients, we studied the erythrocyte 2,3-DPG which regulates oxygen transport function. The concentration of erythrocyte 2,3-DPG was determined in 27 cases with Duchenne type muscular dystrophy and 10 cases with myotonic dystrophy (MyD). We analyzed the relation of erythrocyte 2,3-DPG to spirogram, arterial blood gas and acid-base analysis in these patients. 14 normal males were used as controls. In control subjects, the mean concentration ratio of 2,3-DPG and hemoglobin (DPG/Hb) was 0.880 +/- 0.072. 18 cases of DMD and 9 of MyD, which showed more than 45 torr of Pco2 in arterial blood gas, revealed 0.823 +/- 0.053 and 0.814 +/- 0.092 of DPG/Hb respectively. These values were significantly lower than that of controls. DPG/Hb correlated to % VC, Pao2, Paco2, pH, HCO3 and BE in DMD, but no relation to these parameters in MyD. The low ratio of DPG/Hb in erythrocyte was considered to be metabolic compensation of respiratory failure in DMD. On the other hand, 2,3-DPG of MyD seemed to be also affected by any other factors in addition to respiratory failure.