The effect on aerosol deposition from a pressurised metered dose inhaler of a 750 cm3 spacer device with a one way inhalation valve (Nebuhaler, Astra Pharmaceuticals) was assessed by means of an in vivo radiotracer technique. Nine patients with obstructive lung disease took part in the study. The pattern of deposition associated with use of a metered dose inhaler alone was compared with that achieved with the spacer used both for inhalation of single puffs of aerosol and for inhalation of four puffs actuated in rapid succession and then inhaled simultaneously. On each occasion there was a delay of 1 s between aerosol release and inhalation, simulating poor inhaler technique. With the metered dose inhaler alone, a mean (SEM) 8.7 (1.8)% of the dose reached the lungs and 80.9 (1.9)% was deposited in the oropharynx. With single puffs from the spacer 20.9 (1.6)% of the dose (p less than 0.01) reached the lungs, only 16.5 (2.3)% (p less than 0.01) was deposited in the oropharynx, and 55.8 (3.1)% was retained within the spacer itself. With four puffs from the spacer 15.2 (1.5)% reached the lungs (p = 0.02 compared with the metered dose inhaler alone, p less than 0.01 compared with single puffs from the spacer), 11.4 (1.2)% was deposited in the oropharynx, and 67.5 (1.8)% in the device itself. It is concluded that the spacer device gives lung deposition of metered dose aerosols comparable to or greater than a correctly used inhaler and oropharyngeal deposition is greatly reduced. The spacer should be used preferably for the inhalation of single puffs of aerosol but may also be used for the inhalation of up to four puffs actuated in rapid succession and then inhaled simultaneously.