The construction and specific function of a new ultrasonic flowmeter are described. The mean velocity of the respiratory airflow is calculated by measuring the transit times of short ultrasonic pulse trains, simultaneously transmitted upstream and downstream at a 500-Hz rate. The flowmeter system consists of a control unit and a separate flow head. The former includes the power supplies, a controlling microprocessor, most of the signal-processing circuitry, and three analog outputs for flow, volume, and temperature. The flow head contains the respiratory tube with a constant circular cross section (length 90 mm, diam 20 mm, dead space 35 ml), a fast temperature sensor, two electronic circuits for processing of flow and temperature data, and a sound transmission channel with two capacitive ultrasonic wide-band transducers. This respiratory airflow meter, suitable for spirometric maneuvers (vital capacity, forced vital capacity) as well as for long-term breath-by-breath respiratory analysis, is extremely fast (response time 1-2 ms) and accurate (volume accuracy with room air +/- 0.7%), with low noise (below 9 ml/s), a wide flow range (bidirectional from 0 to 9 l/s), and a flat frequency response up to 70 Hz.