OBJECTIVE To determine whether changing heat and moisture exchangers every 48 hrs rather than 24 hrs would affect their efficacy to preserve heat and moisture of expiratory gases. METHODS Prospective, controlled, randomized, not blinded, study. METHODS Intensive care unit of a university hospital. METHODS Twenty-nine patients requiring controlled mechanical ventilation and paralysis for >2 days. METHODS After randomization, the patients were allocated to one of the three following groups: a) group 1, ventilated for 24 hrs with a heat and moisture exchanger; b) group 2, ventilated for 48 hrs with the same heat and moisture exchanger; and c) group 3, ventilated for 48 hrs with a heated humidifier system. RESULTS In each patient, during the inspiration phase, the following measurements were performed: a) peak and mean airway pressures; b) mean values of temperature; c) relative and absolute humidity of inspired gases. In each patient, measurements were performed after 24 hrs and after 48 hrs, where appropriate. After 24 hrs, patients in groups 1 and 2 had similar levels of temperature (30.1 +/- 2.7 degrees C and 29.2 +/- 2.3 degrees C), relative humidity (98.3 +/- 3.6% and 99.3 +/- 3.4%), and absolute humidity (29.1 +/- 2.1 and 29.3 +/- 2.4 mg H2O/L). Using the same heat and moisture exchanger for 48 hrs rather than 24 hrs did not affect its technical performance. Results showed the following: a) temperature, 24 hrs, 29.2 +/- 2.3 degrees C, 48 hrs, 28.7 +/- 1.9 degrees C; b) relative humidity, 24 hrs, 99.3 +/- 3.4%, 48 hrs, 99.2 +/- 1.7%; and c) absolute humidity, 24 hrs, 29.3 +/- 2.4 mg H2O/L, 48 hrs, 28.7 +/- 3.1 mg H2O/L. Peak and mean airway pressures did not change over the 48-hr study period, with identical tidal and minute volumes in the study patients. Higher levels of temperature and absolute humidity of inspired gases were observed in group 3, compared with groups 1 and 2 (p< .02). CONCLUSIONS Changing the heat and moisture exchanger after 48 hrs rather than 24 hrs did not affect its technical performance in terms of heat and water preservation of ventilatory gases. There is also some indirect evidence of very few, if any, changes in heat and moisture exchanger resistance. However, other large clinical trials should be undertaken to confirm the safety of extending the time between heat and moisture exchanger change. The heated humidifier, supplied with electric energy maintained high levels of humidification and temperature over the 48-hr study period.