This study examined the effect of several different high-frequency jet ventilation (HFJV) humidification techniques on tracheobronchial mucosa. Six groups (2 in each group) of mongrel dogs, chronically instrumented, were cared for in an intensive care-like setting for data acquisition. Four groups received HFJV with different humidification systems during 72 h of continuous ventilation. Two groups served as controls (conventional ventilation with and without humidity). Although there was significant damage to tracheal mucosa during ventilation without humidification, there were no significant pathologic differences in bronchial mucosa between humidified and nonhumidified groups. Several techniques of HFJV humidification produced no pathologic evidence of mucosal damage and could be clinically useful.