Evidence of the coexistence of asthma and allergic rhinitis has been broadly discussed in the literature. OBJECTIVES: In this study, patients with both conditions were studied and the hypothesis that treating allergic rhinitis reduces health care utilization for co-morbid asthma was tested. METHODS: The study design was a retrospective cohort study using 1994-1995 MarketScan? claims data. The cohort was limited to patients age 12-60 who were continuously enrolled and had no evidence of chronic obstructive pulmonary disease. Allergy treatment and asthma events (hospitalizations and emergency room visits) were identified. To estimate the magnitude of the association between allergy treatment and asthma events, we calculated an incidence density ratio (IDR) associated with exposure to allergic rhinitis treatment. 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. RESULTS: The final study population consisted of 4,944 allergic asthmatics approximately 73 percent of whom were treated for their allergies. Asthma related events were more frequent in the untreated group compared to the treated, 6.6 percent compared to 1.3 percent. An IDR of 0.49 for the treatment group (p=0.001) indicates that the risk of an asthma related event for the treated group was about half that of the untreated group. CONCLUSION: Overall, we found that those who were treated for allergic rhinitis have a significantly lower risk of subsequent asthma related events (ER visits or hospitalizations) than those who were not treated.