Six commonly available heat and moisture exchangers (HMEs) were tested according to a draft ISO standard for evaluating these devices (ISO = The International Organization for Standardization). The devices tested were: Pall Ultipor Filter, Mallinckrodt Inline, Siemens Servo 152, Engström Edith, Triplus Icor, and Portex Humid Vent 1. The trial period was 24 h and three different ventilator settings were used. For each device the following were determined: humidity-conserving ability, heat-conserving ability, resistance to air flow, internal volume, and gas leakage. The water loss (mg/l) at a common ventilator setting (10 l/min, 20.min-1) was as follows: Pall 10.8, Inline 7.5, Servo 9.0, Edith 6.6, Icor 6.2, and for Humid Vent 13.9, as compared to a control value (= no HME) of 24.8. The temperature differences (degrees C) between exhaled and inhaled gas at the patient port of the HME were: Pall 2.39, Inline 1.31, Servo 1.21, Edith 1.40, Icor 1.12, and for Humid Vent 2.80 as compared to a control value of 5.34. Ventilator settings with higher tidal volumes generally resulted in decreased efficiency. Resistance to air flow was less than 3 hPa.l-1.s-1 for all devices tested. The internal volumes ranged from 11 to 87 ml. The gas leakage was zero for all devices. Based on our findings the HMEs could be divided into three groups: 1) Icor, Servo, Inline, Edith: very good performance, 2) Pall: good performance for tidal volumes up to about 0.7, 1, and 3) Humid Vent 1: acceptable performance for tidal volumes up to 0.5 l.