A double-blind, parallel-group, multi-centre study was carried out in 248 patients with symptomatic seasonal allergic rhinitis to assess the effectiveness and tolerability of intranasal aqueous budesonide given as a single daily dose each morning of 400 micrograms compared with the conventional dosage regimen of 200 micrograms twice daily. After a 1-week run-in period during which only oral terfenadine was allowed for intolerable symptom relief, symptomatic patients were allocated at random to receive budesonide in one or other dosage regimen for 3 weeks. The results of assessments made by the physician at clinic visits and by patients recording daily data on diary record cards showed that specific nasal symptom incidence and severity were significantly (p less than 0.001) reduced in both treatment groups. The proportions of patients symptom-free at 3 weeks were 40% in the 400 micrograms once daily and 45% in the 200 micrograms twice daily group; in addition, mean nasal symptom scores from the daily diary cards were significantly (p less than 0.001) reduced in both groups, with a reduction in total symptom scores of 53% and 60%, respectively. The differences between the groups were not statistically significant. Total symptom scores were significantly (p less than 0.01) reduced in both dosage groups at all levels of pollen exposure. Patients rated treatment overall as being highly effective, mean scores being very similar in both groups, and tolerability was similar and good whether budesonide was given as a 400 micrograms once daily dose or as 200 micrograms twice daily. Assuming equal symptom control, 74% of patients stated they would prefer once daily to twice daily treatment.