Hyperventilation during phonation is one of the causes of fatigue in patients with vocal cord disorders. Transcutaneous (TC) PCO2 and PO2 were analyzed during phonation in patients with known recurrent nerve paralyses (RNP). There was no significant change in TCPCO2 in eight normal subjects. In cases with unilateral RNP and incomplete glottic closure, TCPCO2 decreased during phonation. This decrease in TCPCO2 resulted from hyperventilation during phonation. The TCPCO2 did not decrease in one case with RNP and complete glottic closure. In one case with incomplete glottic closure, the decrease in TCPCO2 and fatigue disappeared after treatment. These results suggest that hyperventilation is one of the causes for easy fatiguability during phonation in patients with RNP, Findings also show that measurements of TCPCO2 during phonation are useful for evaluating the cause of fatigue in patients with vocal disorders.