Asthma and allergic rhinitis often coexist and it is suggested that the second-generation H1 antihistamines may be a promising treatment for patients with allergic rhinitis and asthma. This study was conducted to investigate the efficacy and safety of oral cetirizine in children with coexisting allergic rhinitis and asthma. 552 children, mean age 4.8 years, were randomized to receive daily oral cetirizine dihydrochloride drops plus inhaled corticosteroids (intervention group) or inhaled corticosteroids alone (control group) for 12 weeks. Allergic rhinitis symptoms, recent asthma control, the occurrence of acute asthma attacks, use of rescue medications, and adverse events were recorded. The proportions of subjects with good asthma control in the intervention group were 90.5%, 90.7% and 95.6% at Weeks 12, 16 and 24, respectively, which were significantly higher than the proportions in the control group (61.3%, 68.5% and 78.2%, respectively). The proportions of subjects with asthma attacks were significantly lower in the intervention group than the control group at Weeks 12 and 16. The use of rescue medications was significantly less in the intervention group than in the control group. Nasal symptoms improved in both groups and the improvements were significantly more pronounced in the intervention group. Treatment with cetirizine in addition to inhaled corticosteroids significantly improved asthma control and reduced nasal symptoms in children compared to inhaled corticosteroids alone.