Chronic sinonasal diseases (CSDs) are common comorbidity of asthma. The aim of this study was to assess comprehensively CSD in a population of asthma patients and determine whether the clinical factors in both diseases were related to each other. Eighty adult stable asthmatic patients with sinonasal symptoms (SNSs) of 3 months and who were nonresponsive to aggressive medical treatment were prospectively investigated. All patients underwent a detailed ear, nose, and throat examination and were evaluated by paranasal sinus computed tomography (PNS-CT). The severity of asthma was compared with SNS scores (SNSSs) and sinonasal involvement on PNS-CT. Asthma was severe in 12.5%, moderate in 55%, and mild in 32.5% of cases. There were 15 (18.7%) patients in stage 0, 16 (20%) patients in stage 1, 21 (26.2%) patients in stage 2, and 28 (35%) patients in stage 3 according to PNS-CT scoring. There was no correlation between asthma severity and CT stages, total opacification scores (TOSs), anatomic variations, and SNSSs. Although SNSSs were similar in patients with nasal h (NPs) and without NPs, patients with NPs showed significantly higher TOSS and CT stages (p < 0.05). The presence of NPs and age and duration of disease were related to severity of asthma (p < 0.05). Nonatopic asthmatic patients had both more severe asthma (p = 0.05) and more extensive CT findings (p = 0.01). The use of clinical symptoms alone is not very reliable in predicting the presence or severity of CSD. Therefore, SNSs should be supported with objective: criteria such as nasal endoscopy and PNS-CT scan. Furthermore, sinonasal involvement may exist independently from severity of asthma.