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Although asthma and allergic rhinitis commonly occur together, the nature of the association has yet to be determined. Treatments for one condition could potentially alleviate the coexisting condition.Patients with both allergic rhinitis and asthma were studied to test the hypothesis that treating allergic rhinitis reduces health care utilization for co-morbid asthma.A retrospective cohort study was carried out with 1994-1995 MarketScan claims data. The cohort was limited to patients with both allergic rhinitis and asthma, aged 12 to 60 years, who were continuously enrolled and had no evidence of chronic obstructive pulmonary disease. Allergic rhinitis treatment and asthma-related events (hospitalizations and emergency department visits) were identified. An incidence density ratio (IDR) associated with exposure to allergic rhinitis treatment was calculated. A multivariate Poisson regression was estimated, and the parameter estimates were transformed into IDRs for each explanatory variable. An allergic rhinitis treatment indicator was included in all regressions.The study sample population consisted of 4944 patients with allergic asthma, approximately 73% of whom were treated for their allergic rhinitis. Asthma-related events occurred more often for the untreated group compared with the treated group, 6.6% compared with 1.3%. An IDR of 0.49 for the treatment group (P= .001) indicates that the risk of an asthma-related event for the treated group was about half that for the untreated group.In summary, those who were treated for allergic rhinitis have a significantly lower risk of subsequent asthma-related events (emergency department visits or hospitalizations) than those who were not treated.