This study was designed to measure improvement in quality of life of patients with asthma, using a standardized disease-specific questionnaire, the St George's Respiratory Questionnaire, in a year long double-blind, placebo-controlled, group comparative study with nedocromil sodium. Two other questionnaires were used: the Sickness Impact Profile (a measure of general health) and the Hospital Anxiety and Depression Scale. Measurements were made at baseline and following 24 and 48 weeks of treatment. Response to therapy was also evaluated using daily diary card and peak flow measurements, clinic assessments and spirometry. Following a 4 week baseline, 719 adult asthmatics were randomized to treatment with 4 mg nedocromil sodium or placebo. Patients currently maintained on inhaled corticosteroids received treatments four times daily, those on bronchodilator alone received treatments twice daily. The Impacts component of the St George's Respiratory Questionnaire was significantly improved in patients receiving nedocromil sodium, as were night-time asthma, asthma severity at clinic, and daytime inhaled bronchodilator use. In patients receiving placebo, most of the traditional variables improved, and all three questionnaires recorded significant improvements in health. Patients and clinicians judged nedocromil sodium more effective than placebo. The improvement in St George's Questionnaire score in the nedocromil sodium treated patients was approximately double the change considered to be clinically significant. The study has shown that improvements in health with prophylactic therapy for asthma may be quantified by the use of a standardized disease-specific questionnaire.