Background Previous cross‐sectional studies have suggested an association between asthma and attention‐deficit/hyperactivity disorder (ADHD), but the temporal relationship was not determined. Using a nationwide population‐based prospective case–control cohort study (1:4, age‐/gender‐matched), we hypothesized that asthma in infanthood or early childhood would increase the risk of ADHD in later life. Methods In all, 2,294 children with asthma and 9,176 controls aged between 0 and 3 years in 2000 were included in our study. Cases of ADHD that occurred to the end of follow‐up (31 December 2010) were identified. Results Children with asthma had a higher incidence of developing ADHD (7% vs. 4.6%, p < .001) than control cohort during the follow‐up period. After adjusting for age at enrollment, gender, level of urbanization, and comorbid allergic diseases (allergic rhinitis and atopic dermatitis), children with asthma had an elevated risk (HR: 1.31, 95% CI: 1.07–1.59) of developing ADHD compared with control group. Discussion Our prospective study supported a temporal relationship between asthma and ADHD. Asthma in very early life increased the risk of developing ADHD during the school years. Further studies are required to investigate whether the prompt treatment of asthma and comorbid allergic diseases could prevent the development of ADHD or decrease ADHD symptoms.