Background Chronic rhinosinusitis with nasal polyps (CRSwNP) is a common inflammatory disorder associated with asthma. This association is well described in patients with CRSwNP undergoing endoscopic sinus surgery (ESS); however, some patients are never referred for surgery, and the frequency of asthma in this group is largely unknown. Objective To determine the frequency of asthma in patients with CRSwNP treated in a primary care (PC) setting who have never been referred for surgery and to compare this with ESS patients. Methods Fifty-seven patients with CRSwNP who had never undergone ESS were prospectively recruited from nine PC ear, nose, and throat clinics in the Copenhagen area. CRSwNP was diagnosed according to the European Position Paper on Chronic Rhinosinusitis and Nasal Polyps; severity was assessed by using a visual analog scale. Allergy, lung function, and asthma tests (reversibility to β2-agonist, peak expiratory flow variability, and mannitol challenge) were performed. Findings were compared with our previously published data from patients with CRSwNP referred for surgery. Results Asthma was diagnosed in 25 patients (44%) based on respiratory symptoms and a positive asthma test; of these, 12 (48%) had undiagnosed asthma prior to study onset. Furthermore, when using the same methods, we found a lower frequency of asthma in PC patients compared with ESS patients (44% versus 65%, p = 0.04). Conclusion A high prevalence of asthma in PC patients with CRSwNP was found. Frequently, asthma was undiagnosed. However, asthma was significantly less prevalent in PC patients compared with patients referred for ESS. The frequent concomitance of asthma, i.e., united airways disease, in PC patients calls for closer collaboration between ear, nose, and throat specialists, and asthma specialists.