In a double blind, controlled trial, the effect of high dose beclomethasone dipropionate (750 micrograms three times daily for five days) administered by metered dose inhaler and valved spacer, was compared with placebo, during 70 paired episodes of acute asthma in 24 preschool children. Treatment commenced at home at the first sign of an attack. Parents' blind preference for active treatment was significant. Data from 17 pairs of treatment, however, were affected by interventions such as hospital admission or oral corticosteroid treatment. These events occurred similarly in active and control periods. An intrasubject comparison was made of diary scores from the 18 pairs of episodes in which no intervention occurred in either the active or placebo treatment. Both daytime and night symptoms over the first week of the attack were significantly reduced by active treatment. Intermittent high dose inhaled beclomethasone dipropionate is beneficial in modifying the severity of acute episodic asthma in preschool children able to use a spacer device.