A nonlinear flow measurement device was developed for clinical use for long-term monitoring of tidal or minute volume. The device was developed as an alternative to present devices, which require heating and are subject to problems with occlusion by condensation or deposition of particles. Its accuracy was determined in the laboratory by comparison with the Fleisch pneumo-tachograph under conditions of high humidity, temperature change, and change from air to oxygen or nitrous oxide. The adult device was used clinically and the volumes were compared with those obtained routinely with a gas meter. A smaller pediatric device was also used clinically, but no comparison with a gas meter was possible. The laboratory results showed that the device was affected not at all by humidity or temperature change, minimally by varying oxygen concentrations, but to a greater extent by varying nitrous oxide concentrations. Minute volume values obtained clinically with the adult device showed good agreement in general with those obtained using a gas meter. Although no comparison was made for the pediatric device, volumes were obtained continuously for as long as 18 hours with no problem of condensation or occlusion. The conclusion is that the nonlinear device represents a viable alternative to present methods of long-term monitoring of tidal or minute volume.