Eighty-six bilateral impedance audiometries were performed weekly in 35 unconscious ICU patients. Tympanometry showed the presence of middle-ear effusion in 43.3% of the examinations (80% of the patients). The incidence of effusion was significantly higher in patients older than 50 years, in intubated and mechanically ventilated patients and in the presence of microbial colonization of the upper airways. The acoustic reflex (AR) was characterized by increased threshold values in 23.8% of the examinations and was absent in 41.3%. Abnormal or absent ARs were usually associated with middle-ear effusion or tube malfunction.