Although heated humidifiers are considered the most efficient humidification devices for mechanical ventilation, endotracheal tube occlusion caused by dry secretions has been reported with heated-wire humidifiers. We tested the hypothesis that inlet chamber temperature, influenced by ambient air and ventilator output temperatures, may affect humidifier performance, as assessed by hygrometry. Hygrometry was measured with three different humidifiers under several conditions, varying ambient air temperatures (high, 28-30 degrees C; and normal, 22-24 degrees C), ventilators with different gas temperatures, and two VE levels. Clinical measurements were performed to confirm bench measurements. Humidifier performance was strongly correlated with inlet chamber temperature in both the bench (p < 0.0001, r2 = 0.93) and the clinical study. With unfavorable conditions, absolute humidity of inspired gas was much lower than recommended (approximately 20 mg H2O/L). Performance was improved by specific settings or new compensatory algorithms. Hygrometry could be evaluated from condensation on the wall chamber only when ambient air temperature was normal but not with high air temperature. An increase in inlet chamber temperature induced by high ambient temperature markedly reduces the performance of heated-wire humidifiers, leading to a risk of endotracheal tube occlusion. Such systems should be avoided in these conditions unless automatic compensation algorithms are used.