Objective. To determine the incremental effect of allergic rhinitis on health care resource use in children with asthma. Design. Population-based historical cohort study. Setting. Data in a general practice database in the United Kingdom during 1998 to 2001. Patients. Children 6 to 15 years old with asthma and with ≥1 asthma-related visits to a general practitioner (GP) during a 12-month follow-up period. Main Outcome Measures. Asthma-related hospitalizations, GP visits, and prescription drug costs during the 12-month follow-up period for patients with and without comorbid allergic rhinitis. Results. Of 9522 children with asthma, 1879 (19.7%) had allergic rhinitis recorded in the GP medical records. Compared with children with asthma alone, children with comorbid allergic rhinitis experienced more GP visits (4.4 vs 3.4) and more of them were hospitalized for asthma (1.4%vs 0.5%) during the 12-month follow-up period. In multivariable regression analyses, comorbid allergic rhinitis was an independent predictor of hospitalization for asthma (odds ratio: 2.34; 95%confidence interval CI : 1.41-3.91) and was associated with increases in the number of asthma-related GP visits (mean increase: 0.53; 95%CI: 0.52-0.54) and asthma drug costs (mean increase ￡: 6.7; 95%CI: 6.5-7.0). The association between allergic rhinitis and higher costs of prescriptions for asthma drugs was independent of asthma severity, measured indirectly by the intensity of use of asthma drugs. Conclusions. Children with comorbid allergic rhinitis incurred greater prescription drug costs and experienced more GP visits and hospitalizations for asthma than did children with asthma alone. A unified treatment strategy for asthma and allergic rhinitis, as recommended by the Allergic Rhinitis and Its Impact on Asthma initiative, might reduce the costs of treating these conditions. Objective. To determine the incremental effect of allergic rhinitis on health care resource use in children with asthma. Design. Population-based historical cohort study. Setting. Data in a general practice database in the United Kingdom during 1998 to 2001. Patients. Children 6 to 15 years old with asthma and with ≥1 asthma-related visits to a general practitioner (GP) during a 12-month follow-up period. Main Outcome Measures. Asthma-related hospitalizations, GP visits, and prescription drug costs during the 12 Of 9522 children with asthma, 1879 (19.7%) had allergic rhinitis recorded in the GP medical records. Compared with children with asthma alone, children with comorbid allergic rhinitis experienced more GP visits (4.4 vs 3.4) and more of them were hospitalized for asthma (1.4% vs 0.5%) during the 12-month follow-up period. In multivariable regression analyzes, comorbid allergic rhinitis was an independent predictor of hospitalization for asthma (odds ratio: 2.34; 95% confidence interval CI: 1.41-3.91) and was associated with increases in the number of asthma-related GP visits (mean increase: 0.53; 95% CI: 0.52-0.54 ) and asthma drug costs (mean increase £: 6.7; 95% CI: 6.5-7.0). The association between allergic rhinitis and higher costs of prescriptions for asthma drugs was independent of asthma severity, measured indirectly by the intensity of use of asthma drugs . Conclusions. Children with comorbid allergic rhinitis greater drug prescription and experienced more GP visits and hospitalizations for asthma than did children with asthma alone. A unified treatment strategy for asthma and allergic rhinitis, as recommended by the Allergic Rhinitis and Its Impact on Asthma initiative, might reduce the costs of treating these conditions.