BACKGROUND: A study was undertaken to test the hypothesis that airway calibre may alter lung deposition and therefore lung bioavailability of inhaled drugs as a result of narrowed airways reducing peripheral drug delivery. This was evaluated using the early lung absorption profile of salbutamol over the first 30 minutes after inhalation. METHODS: Three groups were compared: (1) 10 normal subjects with mean forced expiratory volume in one second (FEV1) 109.5% predicted and mid forced expiratory flow (FEF25-75) 103.0%, (2) 10 mild asthmatic patients with FEV1 102.0% and FEF25-75 82.6%, and (3) 10 severe asthmatic patients with FEV1 49.2% and FEF25-75 27.5% predicted. Each subject had one study visit where a single dose of nebulised salbutamol was given (40 micrograms/kg) via a Ventstream nebuliser with mouthpiece followed by mouth rinsing. Plasma salbutamol levels were measured at five, 10, 20, and 30 minutes after the end of nebulisation with calculation of maximal (Cmax) and average (Cav) concentration over 0-30 minutes. Systemic beta 2 responses (plasma potassium, tremor and heart rate) and airway responses (FEV1, FEF25-75) were measured before and 30 minutes after nebulisation. RESULTS: For Cav over 0-30 minutes the severe asthmatic patients had a lower plasma salbutamol concentration (1.31 ng/ml) than either the normal subjects (2.40 ng/ml) or those with mild asthma (2.45 ng/ml): normal subjects versus severe asthmatics 95% CI 0.30 to 1.88, mild versus severe asthmatics 95% CI 0.07 to 2.21. Airway responses as delta FEF25-75 were lower in the severe asthmatic subjects (0.30 l/s) than in either the normal subjects (0.69 l/s) or those with mild asthma (0.74 l/s): normal subjects versus severe asthmatic subjects 95% CI 0.09 to 0.88, mild versus severe asthmatics 95% CI 0.04 to 0.93. Values for delta log tremor also showed attenuated responses in those with severe asthma (1.22 mg2/s) compared with normal subjects (2.00 mg2/s) or those with mild asthma (2.02 mg2/s): normal subjects versus those with severe asthma 95% CI -0.02 to 3.30, mild versus severe asthmatics 95% CI 0.02 to 3.30. CONCLUSIONS: These results show that baseline airway calibre significantly alters the early lung absorption profile of salbutamol in patients with severe asthma. This may have implications in terms of optimising dose and delivery of inhaled beta 2 agonists in these patients.