In a recent clinical study we have demonstrated that the bronchodilator effect of 200 microg salbutamol (Ventoline) was spacer device-dependent in 100 tested asthmatic children, with the Babyhaler providing greater efficacy for improving peak expiratory flow rate compared to Aeroscopic, Nebuhaler, Aerochamber and Volumatic. The aim of this present study was to correlate our clinical results to in vitro determinations of the emitted dose (ED) of Ventoline administered via these five different plastic spacer devices. ED was determined from the mean of single doses collected in unit dose sampling tubes using a constant suction flow of 28.3 L/min. Three pressurized metered-dose inhalers and three sets of spacer devices were used to obtain a total of 30 measurements per group. Inter-group results were compared by RM-ANOVA or Student-Newman-Keuls method when indicated. Babyhaler delivered significantly (P < 0.05) more salbutamol than Nebuhaler, Aerochamber and Aeroscopic (mean +/- standard deviation: 63.6 +/- 2.9 microg/100 microg actuation for Babyhaler vs. 59.4 +/- 8.6 for Nebuhaler, 50.8 +/- 5.0 for Aerochamber and 47.5 + 2.5 for Aeroscopic). The ED from Volumatic (61.5 +/- 7.9 microg/100 microg actuation) was similar to that from the Babyhaler. The variability in the ED was greatest with the large volume spacers. Despite a greater ED from the Babyhaler, in vitro results do not fully explain the in vivo results. However, the previously described clinical improvement seen with the Babyhaler may be due to the quantitatively different aerosol production in a more 'useful' size range, as well as the different breathing patterns of the children tested. The results of this present study question the relevance of mouthpiece filter collection studies using a constant sampling in predicting clinical or physiological outcomes.