Under-humidification and associated complications may occur with heated humidifiers. Hygrometric performances of heated wire humidifiers are reduced by high ambient and high outlet ventilator temperatures. Currently, there is no reliable monitoring tool to evaluate humidification performances of heated wire humidifiers in the daily practice. We sought to demonstrate the relation between humidity delivered by heated wire humidifiers and different parameters that could be used to monitor humidity of gas delivered to subjects. On a bench test, we measured heater plate temperature, inlet chamber temperature, and delivered humidity with MR850 system. Temperature displayed on the humidifier was also recorded. The measurements were performed at different ambient temperatures and five minute ventilation levels (5, 7.5, 10, 12.5, 15 L/min). Inlet chamber temperatures varied from 20-40°C. In each condition, hygrometric measurements with the psychrometric method were performed at steady state. We performed 279 measurements at steady state including all conditions. We found a good correlation between heater plate temperature and absolute humidity delivered (R2 = 0.82). This relationship was hardly affected by ambient temperature, but minute ventilation had more effect. For different minute ventilations, the correlation between heater plate temperature and absolute humidity delivered was very good with coefficient of determination R2 from 0.87-0.98. Heater plate temperature > 62°C was a good predictor of absolute humidity delivered > 30 mg H2O/L (area under the curve = 0.96, sensibility 79%, specificity 94%). No correlation existed between humidity delivered and the outlet chamber temperature (displayed on the humidifier). In this bench study, we have shown a good correlation between heater plate temperature and humidity delivered with a heated wire humidifier. This means that a "hidden" hygrometer is built into the heated wire humidifier. Heater plate temperature should be used as a surrogate of humidity to improve the humidification monitoring.