In vitro studies with the Diskus inhaler at low and high flow rates show consistent doses of drug as fine particles <4.7 microm. The present study was designed to ascertain whether this in vitro flow independency translates into flow-independent clinical effect when the device is used by patients at low (30 L x min[-1]) and high (90 L x min[-1]) flow rates. A pilot study in 129 children aged 3-10 yrs demonstrated that 99% of children of 3 yrs and above can generate a flow > or = 30 L x min(-1) through the device, while 26% performed > or = 90 L x min(-1). Eighteen children aged 8-15 yrs with exercise induced asthma inhaled placebo or salmeterol 50 microg at either 30 L x min(-1) or 90 L x min(-1). Exercise challenges were carried out 1 h and 12 h after dosing. The maximum percentage fall in forced expiratory volume in one second (FEVI) after exercise 12 h after treatment was significantly less after salmeterol at either flow rates as compared to placebo. There was no significant difference in the protection from salmeterol on the day of low-flow inhalation versus the day of high-flow inhalation. Consistent in vitro fine particle dosing from the Diskus inhaler translates into a consistent clinical effect at low and high flow rates in children.