Allergic rhinitis (AR) is the most common chronic condition in pediatric populations. Characteristic symptoms in AR may bother daily activities and disturb sleep, leading to daytime inattention, irritability, and hyperactivity, which are also components of attention deficit/hyperactivity disorder (ADHD). Conflicting data exist in the literature regarding the relationship between ADHD and AR. The aim of this nationwide population-based study was to examine the prevalence and risk of ADHD among AR patients in a pediatric group. Data from a total of 226,550 pediatric patients <18 years old were collected from Taiwan's National Health Insurance Research Database from January 1 to December 31, 2005 and analyzed. We calculated the prevalence of allergic diseases based on various demographic variables, as well as in ADHD patients. We also used multivariable logistic regression to analyze the risk factors of ADHD. In 2005, the period prevalence rates of atopy and ADHD in patients <18 years of age were 15.35 and 0.6%, respectively. Pediatric patients with AR had a substantially increased rate of ADHD (p < 0.001) in terms of period prevalence and odds ratio. This significance existed across various demographic groups regardless of age, gender, area, or degree of urbanization. Neither comorbidity of atopic dermatitis nor bronchial asthma carried high risk for ADHD in AR patients. The present study revealed an increased rate of ADHD among AR patients. Therefore, evaluation of ADHD is advised for treatment of AR children.