This open randomized, cross-over study compared the clinical efficacy and patient acceptability of the two bronchodilator delivery systems, terbutaline Turbuhaler (0.5 mg t.i.d.) and salbutamol Rotahaler (0.4 mg t.i.d.), each given for 3 weeks. Thirty-two adult asthmatics (21 males and 11 females with a mean age of 34 years) who demonstrated at least 15% reversibility in PEF or FEV1 in response to terbutaline, were enrolled for study. The median reversibility in FEV1 was 27.5% for the terbutaline-salbutamol group and 21% for the salbutamol-turbutaline group. Two patients discontinued during terbutaline treatment (one due to respiratory infection and one due to tachycardia, exhaustion and tremor) and five patients were lost to follow-up during salbutamol treatment, leaving data from 25 patients for an 'all patients treated' analysis. Mean morning PEF was 426 l min-1 during terbutaline and 410 l min-1 during salbutamol (difference 16 l min-1, 95% CI of difference 3-28 l min-1, P = 0.016), and mean evening PEF was 446 l min-1 during terbutaline and 428 l min-1 during salbutamol (difference 18 l min-1, 95% CI 5-30 l min-1, P = 0.0076). No significant differences were detected in diary symptom scores or in use of additional study drug during the day or night, and no serious adverse events were reported. When asked to state their treatment preferences on the basis of effects, side-effects and overall, more patients preferred Turbuhaler in each case, although no statistically significant differences were detected. In conclusion, terbutaline via Turbuhaler was significantly more effective than salbutamol via Rotahaler in controlling lung function (mean daily PEF) in adults with mild to moderate asthma, and it was the preferred treatment overall in 44% of patients, compared with 16% for Rotahaler (n.s.).