This study was designed to compare the bronchodilatatory effect of terbutaline inhaled through Turbuhaler (TH) or pressurized metered-dose inhaler (pMDI) in young asthmatics, and to assess the possible relationship between patients' inspiratory capacity and bronchodilatation for both devices. One hundred and eighteen asthmatics (aged 4 10/12-20 6/12 years) with bronchial obstruction (mean Vmax 50%: 59.5% pred, SD 17.8% pred) were allocated at random to two groups of 59 patients to inhale 0.5 mg terbutaline either by TH or by pMDI (and placebo by dummy of the other device). In- and expiratory spirometry and bodyplethysmography were conducted before and 10 min after inhalation. Bronchodilatation was effective [change in airways resistance (delta RAW) -50%, change in forced expiratory volume in 1 s (delta FEV1)+15%, delta Vmax 50% or 25% + 25% of baseline] in 41 of 59 patients with pMDI (69.5%) and 33 of 59 patients with TH (55.9%). The effect on Vmax 50% was significantly better with pMDI than with TH. Turbuhaler users with higher inspiratory flow [forced inspiratory volume in 1 s (FIV1), forced inspiratory flow at 50% vital capacity (FIF50)] reached better bronchodilatation, while bronchodilatatory effect was not correlated with inspiratory performance in MDI users. Peak inspiratory flow (PIF) did not correlate well with bronchodilatation by TH. When using TH for bronchodilatation, the effectiveness of terbutaline depends upon the degree of inspiratory capacity. This can lead to impaired bronchodilatatory effect in subgroups of obstructive young asthmatics with low inspiratory flow. In contrast, when using a pMDI, inspiratory capacity does not seem to influence the effectiveness of terbutaline.