||The concept of asthma and COPD as separate conditions has been questioned, and the term asthma-COPD overlap syndrome has been introduced. We assessed the prevalence, symptoms, and lifestyle factors of asthma-COPD overlap (ACO) in a large Norwegian population-based study. From 2006 to 2008, a total of 50,777 residents of Nord-Trondelag participated in the Nord-Trondelag Health Study, Norway. They completed questionnaires regarding respiratory symptoms, disease status, and medication use. We estimated the prevalence and 95% confidence intervals of ACO. Additionally, spirometry was used to estimate the prevalence of ACO in a subgroup. The prevalence of self-reported ACO was 1.9%, and in age groups <40, 40-60 and >/=60 years it was 0.7%, 1.4%, and 3.2%, respectively. Among those reporting COPD, the proportion of ACO was 0.56. In the spirometry subgroup when ACO was defined as doctor diagnosed asthma ever and FEV1/FVC < 0.70, the prevalence of ACO was 2.0%. All respiratory symptoms, separately or in combination, as well as medication use were reported most frequently in those with ACO compared to the other groups. Strikingly, we observed a two-fold higher proportion of allergic rhinitis in ACO compared to COPD only. In this Norwegian population, the prevalence of self-reported ACO was 1.9%, and the corresponding proportion of ACO among those with COPD was 0.56. Participants with ACO generally had the highest proportions of respiratory symptoms compared to asthma or COPD.