Medical humidifier is disclosed for a respiratory flow path connected directly to the airway of a patient, through which life-supporting air can be channeled. It comprises a compact nest of discrete, elongated air transmitting hollow relatively large bore, relatively thin wall fibers connected in parallel by inlet and outlet connections and having walls of substance permeable to water vapor and impermeable under operating conditions to liquid water. A relatively negative-pressure water-filled chamber surrounds the effective length of the fibers. The fibers are adapted to divide dry inhalation air for a patient which enters said inlet connection into a series of air-flow filaments for humidification by water vapor that permeates the wall of the fibers from the surrounding relatively negative-pressure water. The fibers are provided in size and number sufficient to freely pass under normal air-flow conditions an aggregate air-flow volume corresponding to the life-supporting need of the patient, and to provide effective fiber surface area sized to pass water vapor sufficient to humidify air at normal flow rates substantially to saturation from said relatively negative-pressure water heated to about body temperature. The outlet connection is arranged to collect the thus humidified filaments of air and direct the unified flow into the patient, whereby the medical humidifier can provide full humidification of air directed into a patient in a practical, failsafe, low flow resistance compact unit that can be disposable.