The upper airways warm and saturate inspired air with water vapour. In intubated or tracheotomized patients, this function is replaced either by hot water humidifiers or by heat and moisture exchangers (HMEs). The aim of this study was to quantify the modifications of ventilatory mechanics and patients' work when two different HMEs were added to spontaneously breathing patients. We studied nine consecutive patients with no previous history of chronic obstructive lung disease. All patients had been weaned from mechanical ventilation. They were breathing through devices supplying positive end-expiratory pressure and/or O2 enrichment. Two different HMEs were used: Icor Mediflux 1 and Icor Mediflux 2. These HMEs have identical chemical composition and configuration, but the Mediflux 1 is larger than the Mediflux 2. The humidification of the inspired gases was obtained alternatively by an active humidifier and the two HMEs. Data regarding ventilatory pattern and respiratory mechanics were collected by pulmonary monitor CP100 (Bicore). Tidal volume, work of breathing and pressure-time product were greater with Mediflux 1 than with Mediflux 2 or active humidifier. There were no significant differences in respiratory rate, intrinsic positive end-expiratory pressure (PEEPi), rapid shallow breathing index, arterial CO2 and O2 partial pressure. The larger HME (Mediflux 1) increased patient's effort, with no evidence of patient discomfort. However, the smaller HME (Mediflux 2) did not add a detectable load and provided adequate humidification. In conclusion, the smaller HME appears to be preferable for the management of spontaneously breathing patients.