A high‐resolution infrared optical absorption spectroscopy system based on lead‐salt tunable diode lasers as light sources has been developed for detection and monitoring of trace gases by resolving their narrow‐width absorption bands. The system is a double beam setup, where a reference cell containing the monitored gas is used for tuning and locking the laser light frequency to that of a desired absorption line and a measurement cell where the examined gas flows continuously at a reduced pressure. Frequency modulation of the laser light is achieved by square waveform modulation of its current source and both the frequency locking and signal analysis devices are based on sample‐and‐hold techniques. The system capability is demonstrated by the monitoring of ammonia traces in human breath. Gas trace monitoring in respiration may prove to be an effective medical diagnostic method.