Upper airway dryness is a frequent side-effect of nasal continuous positive airway pressure therapy (nCPAP) in obstructive sleep apnoea (OSA). In this situation, heated or non-heated passover humidifiers are often added to the nCPAP-therapy. The efficacy of these two modes in terms of increasing the absolute humidity of the inspired air in vivo has so far not been established. The present investigation was therefore designed to compare various heated and non-heated passover humidifiers in terms of the their ability to increase the absolute humidity in the inspired air during nCPAP. In six healthy test individuals, nCPAP-therapy at pressures of 5 mbar and 10 mbar was simulated, and the relative humidity and temperature of the air within the tube at the junction between CPAP tube and mask were measured. In each test person, measurements were carried out both with and without the two heated (HC 100, Fischer&Paykel Inc., New Zealand and HumidAire, ResMed Ltd., Australia) and two non-heated (Oasis and Humidifier, both from Respironics Inc., U.S.A.) passover humidifiers under steady-state conditions. The absolute humidity was calculated from the relative humidity and temperature measurements. The mean (SD) absolute humidity (gm(-3)) in the steady-state was significantly (P<0.05 higher with each of the humidifiers than that calculated when no humidifier was used. The relevant figures were as follows: no humidifier: 10(-2) (1.8) gm(-3) (at 5 mbar)/9.8 (1.8) gm(-3) (at 10 mbar); Humidifier: 16.4 (0.97)/15.6 (1.26); Oasis: 17.3 (0.97)/ 16.7 (0.93); HC100: 26.5 (1.40)/26.2 (1.23); HumidAire: 31.8 (2.50)/30.9 (2.64). The mean increase in absolute humidity (in gm(-3)) with the aid of the heated humidifiers was 16.3 (5 mbar) gm(-3)/16.4 (10 mbar) gm(-3) with HC100 and 21.6/21.1 with HumidAire, and in both cases was clearly and significantly (P=0.028) higher in comparison with the non-heated humidifiers--6.2/5.8 with Humidifier and 7.2/6.9 with Oasis. In terms of the absolute humidity achieved within the CPAP tube system, the heated humidifiers were clearly superior to the non-heated humidifiers. These results were, however, obtained under laboratory conditions, and therefore cannot be translated unreservedly to the situation represented by long-term CPAP-treatment. Furthermore, it is possible that the smaller humidification capacity of the non-heated humidifiers may still suffice to meet the requirements of clinical use in terms of effectively preventing dry airways under CPAP treatment. This point, however, needs further investigation on the basis of long-term clinical studies.