OBJECTIVE: The purpose of this research was to determine the clinical relevance of an observed statistical improvement. METHODS: A randomized controlled trial was performed to compare once daily fexofenadine HCl (120 mg and 180 mg combined, n=559) to placebo (n=286) in patients with seasonal allergic rhinitis. The Rhinitis Quality of Life Questionnaire (RQLQ), assessing quality of life in the areas of activities, sleep, miscellaneous symptoms, practical problems, nasal symptoms, eye symptoms and emotions, was administered at baseline and 1 and 2 weeks post treatment. Mean change from baseline differences between treatments, averaged over Weeks 1 and 2, were analyzed as the most powerful test of treatment differences. However, it has been shown that a within-patient change of 0.5 on the 7-point scale of the RQLQ is the minimum difference of clinical relevance (Juniper et al, 1996). This within-patient change was used to interpret statistically significant mean differences. RESULTS: Patients receiving fexofenadine HCl showed statistically significant mean improvements in total rhinitis related quality of life over patients receiving placebo (mean?SE=-0.39 ? 0.05, -0.60 ? 0.03 for placebo and fexofenadine HCl patients respectively; p-value =.0002). The proportion of fexofenadine HCl patients who showed a minimal clinically significant improvement of 0.5 or greater was also higher than patients receiving placebo (p=0.38, 0.48 for placebo and fexofenadine HCl patients respectively, p-value = .0045). CONCLUSIONS: In this study, a statistically significant improvement in total rhinitis related quality of life for patients receiving once daily (120 mg and 180 mg) fexofenadine HCl was also clinically relevant.