OBJECTIVE Airways humidification during mechanical ventilation (MV) can be performed by means of heat and moisture exchangers (HME). Good performance of selected hygroscopic HME was shown in terms of tracheal temperature and humidity control and low tube obstruction rates. However, few data are available on their effects on tracheobronchial ciliated cells. The purpose of this study was to evaluate the effects of 2 different HME on ciliated cell during MV. METHODS Twenty consecutive patients undergoing elective surgery lasting <4 h were randomly assigned to be treated with Rusch (Group 1) or Mediflux (Group 2) hygroscopic HME. The morphology of tracheobronchial ciliated cells was evaluated from cytologic smears of endotracheal aspirate. Smears were prepared by using the thin-prep processor, stained using the Papanicolau method and examined in triplicate: recognizable respiratory cells were graded on a six-point scale, according to the normal appearance of cilia, cytoplasm and nucleus. RESULTS In Group 1, 178.3+37 cells were retrieved from aspirates and , 155.6+58 in Group 2 (NS). The score was 739+241 in Group 1 and 617+329 in Group 2 (NS). Cilia and end plate were undamaged in 80+17% and 81+20% cells respectively in Group 1 and in 56+23% and 62+22% cells respectively in Group 2 (P<0.01 and <0.05). CONCLUSIONS During MV epithelial tracheobronchial cells can be affected by the humidification device used. Our findings suggest that the 2 HME tested in the present study have a different impact on ciliated cells structure.