AIM: The study evaluated the effects of breathing helium-oxygen(79% helioxand 21% oxygen) on respiratory insufficiency by way of experimental andclinical observations. METHODS: Six mongrel dogs were randomly inspiredair or heiox after dripping 2% methacholine in tracheas controlled by 900Cventilator. During the experimental course, respiratory mechanical limits,hemodynamic index and blood gas values were observed. Six ventilated pa-tients with COPD in the respiratory intensive care unit inspired heliox, and thedata of respiratory mechanics were recorded. RESULTS: The results demon-strate that heliox can reduce airway resistance, airway pressure, and work ofbreathing, improve lung effective compliance, but heliox has no side effects onpulmonary circulation and systemic circulation. Heliox improves oxygen dif-fusion and CO2 climinating. CONCLUSION: Breathing heliox is one of theeffective ways for patients with severe obstructive pulmonary diseases.