We conducted a prospective study of 74 adults-34 men and 40 women, aged 18 to 90 (mean: 60.2)-to determine the prevalence of middle ear effusion (MEE) among patients in the setting of an intensive care unit (ICU) and to compare the findings with those of a control group of non-ICU hospitalized patients. Other goals were to identify risk factors associated with MEE in ICU patients and to evaluate any association with fever. Both groups included 37 patients. MEE was present in 19 patients (51.4%) in the ICU group, compared with only 2 patients (5.4%) in the control group (p < 0.01; odds ratio: 18.5; 95% confidence interval: 3.9 to 88.3). In the ICU group, there were statistically significant associations between MEE and both the use of mechanical ventilation (p = 0.03) and the use of sedation (p = 0.02). No significant relationships were seen in terms of length of stay, body position, the use of an endotracheal tube, the length of ventilation, and the use of a feeding tube. Fever was present in 8 ICU patients (21.6%) and 3 controls (8.1%), but none of the fevers was associated with MEE. We conclude that adult ICU patients have a high prevalence of MEE (51.4% in our sample) that is perhaps unrecognized. We believe that MEE in these patients is most likely related to altered consciousness, sedation, and mechanical ventilation. MEE was an unlikely cause of fever.