A heat moisture exchanger (HME) with bacterial filtering capabilities was evaluated over an 8-month period in a total of 170 ICU patients. During this time there were 15 endotracheal tube (ETT) occlusions in 15 patients. Over the ensuing 4 months, cascade humidification was used for 81 patients and only one ETT occlusion occurred (p less than .01). The HMEs were replaced frequently with cascade humidifiers during the evaluation period because of inadequate airway humidification. The increase in ETT occlusion was associated with an increased incidence of pneumonia (p less than .001) and atelectasis (p less than .01). Most patients with ETT occlusion required minute volumes greater than 10 L and F10(2) greater than 0.4. We conclude that HMEs do not provide sufficient airway humidification for generalized ICU use. Their role outside of the operating room remains to be determined.