This study was designed to test whether awareness of the measurement of breathing influences the breathing pattern in healthy subjects under routine laboratory conditions. Seventy four subjects (40 females and 34 males), aged 21-63 yrs, were studied under three different conditions whilst their breathing was being recorded for 5 min by means of inductance plethysmography (Respitrace): 1) subjects were misled into believing that their breathing was not being recorded but that they had to wait for 5 min whilst equipment was calibrated; 2) subjects were instructed that their breathing pattern was being recorded for 5 min; 3) the subject's breathing was recorded for 5 min with mouthpiece and pneumotachograph. The first two conditions were randomized. The Respitrace was calibrated by means of multiple linear regression carried out during the 5 min period of quiet breathing through a mouthpiece. Awareness of the recording of breathing caused prolongation of inspiratory (tI) and expiratory time (tE). Breathing through the mouthpiece resulted in an increase of tI, tE and tidal volume (VT). The breathing irregularities (sighs and end-expiratory pauses) decreased when subjects were aware of the recording of breathing and nearly disappeared when subjects breathed through the mouthpiece. The end-tidal carbon dioxide concentration was not significantly different between the three conditions. Mouthpiece breathing often induced some respiratory discomfort and even anxiety, particularly in females. Awareness by the subject that his or her breathing was being recorded altered the spontaneous breathing pattern, mainly the breathing frequency. In addition, use of a mouthpiece markedly increased tidal volume, particularly in females in whom mouthpiece breathing induced more complaints than in males.