We developed a prototype laser monitor, consisting of a single laser sensor, to observe chest wall displacement during respiration. With this monitor, respiratory waveforms are expressed as an anterioposterior motion of the chest wall. The purpose of this study was to examine the characteristics and performance of this prototype. Performance was assessed: 1) under static conditions; 2) using a lung model ventilated in both conventional and high frequency oscillation (HFOV) modes; and 3) during spontaneous breathing in normal adults. In vitro, the monitor performed well both under static conditions and during mechanical ventilation. Reliable "respiratory" wave forms, with no frequency-dependent change in the relationship between displacement and volume, were produced during both conventional ventilation and HFOV at 15 Hz. In vivo, abdominal displacement, measured in the midline, was linearly correlated with the tidal volume signal integrated from flow. The waveforms produced by the monitor were adequate for monitoring respiration and for calculating respiratory timing variables. While a single laser sensor is unlikely to be sufficient for monitoring respiration in spontaneously breathing subjects, the performance of the prototype monitor was sufficiently impressive to encourage further development and further study of this type of truly noninvasive respiratory monitor.