Allergic rhinitis and asthma each require costly medical resource utilization, and the impact of both conditions is believed to be even greater. This retrospective cost of illness study evaluated the impact of allergic rhinitis on asthma medical care resource utilization rates and costs for patients with asthma plus allergic rhinitis vs. patients with asthma alone. Patients with one or more claims for asthma (n=27,398) were identified from a medical claims database from a large, northeastern U.S. health insurance plan (1992-1994). A subset of 9226 patients also had at least one visit for allergic rhinitis. Inpatient, professional service, major medical, and pharmaceutical (prescription medications) claims were examined. Patients with asthma and allergic rhinitis had greater medical utilization and costs than individuals with asthma only. The presence of allergic rhinitis was associated with greater frequencies and costs of prescriptions for all asthma-related medications evaluated in the study. Patients with both conditions were also more likely to receive care from medical specialists and less likely to be in managed care. Controlling for these factors, allergic rhinitis was still associated with an increase (P<0.0001) in annual costs of more than dollars 350. Allergic rhinitis in patients with asthma nearly doubles annual medical resource utilization and costs and is associated with increased utilization of asthma-related medications. Based on use of asthma medications, patients with concomitant allergic rhinitis can be regarded as having more severe asthma than do those without allergic rhinitis. Physicians should assess asthma patients for symptoms of allergic rhinitis to improve asthma treatment.