The purpose of this study was to evaluate the efficacy of external high-frequency oscillation (EHFO) in patients with hypercapnia following upper abdominal surgery. Seven patients were ventilated with EHFO for 2 hr at 60 oscillations/min, with cuirass pressures of 36 cm H₂O (-26 to +10), and an inspiratory to expiratory ratio of 1 : 1. Blood gases and cardiac functional parameters were examined during the 2 hr on EHFO. Pulmonary functional parameters were analyzed prior to the institution and after the termination of EHFO. PaCO₂ significantly decreased from 61±8 mmHg to 48±7 mmHg after 10 min on EHFO (p<0.01). PaO₂ significantly increased from 74±10 mmHg to 95±26 mmHg after 1 hr on EHFO (p<0.01). The heart rate decreased significantly from 108±27 beats/min to 101±24 beats/min after 30 min on EHFO (p<0.05). The FEV, and FVC significantly increased from 1.09±0.54 L to 1.50 ± 0.46 L (p<0.01) and from 1.90±0.74 L to 2.18±0.60 L (p<0.05), respectively. The other parameters of lung function also significantly improved after the termination of EHFO. The significant changes in all of the pulmonary functional parameters continued for 1 hr after the termination of EHFO. EHFO is an effective method of gas exchange which is associated with earlier return to preoperative lung function. (J Nippon Med Sch 1997; 64:440-445)