A double-blind, parallel group trial of a 1 mg sodium cromoglycate metered dose inhaler, a 5 mg formulation and a placebo aerosol was undertaken in 139 asthmatic patients with extrinsic allergic asthma. None of the patients had previously been treated with sodium cromoglycate and few (15%) were familiar with the use of a pressurized aerosol device. Each test treatment was taken at a dose of 2 'puffs' 4-times per day for a period of 12 weeks. Response to treatment was assessed by analysis of symptom scores, medication usage and 3-times daily home measurements of PEFR recorded on diary cards, and by assessment of asthma severity and lung function in the clinic at the beginning and end of a 2-week baseline period and at intervals of 3 weeks throughout the trial. Analysis of patient-generated data (symptom scores and PEFR) demonstrated statistically significant differences in favour of each active treatment compared with placebo treatment. Clinic assessments of asthma severity and the investigator's opinion of treatment also showed the superiority of each sodium cromoglycate treatment regimen over placebo. These differences were statistically significant after 9 weeks (high dose) and 12 weeks (both doses). No statistically significant differences were demonstrated between the two active treatment groups in the clinic assessments of asthma severity, or in the home or clinic measurements of PEFR. There was, however, a consistent trend in favour of the higher dose formulation with respect to diary card symptom scores. In general, improvement in the low-dose sodium cromoglycate treatment group was slower than in the high-dose group suggesting an advantage for the 5 mg sodium cromoglycate inhaler in terms of onset of therapeutic response. The results indicate that sodium cromoglycate administered by pressurized aerosol at a dose of 2 x 1 mg or 2 x 5 mg 4-times daily is effective in the treatment of asthma and suggest that the higher dose formulation may provide more rapid control of symptoms.