Asthma induced by ingestion of aspirin occurs when symptoms arise within 30 minutes to three hours after aspirin consumption. Previous data indicate that sensitivity to aspirin may be associated with poorly controlled asthma. This study aims to evaluate the frequency of aspirin sensitivity in patients with moderate to severe asthma receiving conventional asthma therapy. This clinical trial was conducted on 65 patients aged 18 to 65 years with moderate to severe asthma from February 2015 to February 2016 at the Allergy Department, Hazrat-e-Rasoul Hospital, Iran University of Medical Sciences, Tehran. To assess treatment responses in patients, forced expiratory volume in the first second (FEV1) and asthma control test (ACT) scores were measured at baseline and after 3 months. The results of the oral aspirin challenge revealed a prevalence of 35.38% for sensitivity to aspirin. Hypersensitivity reactions to aspirin were detected in 60.9% of the patients with moderate asthma and 39.1% of the patients with severe asthma. All patients with positive aspirin challenge tests suffered from rhinosinusitis and in 56.5% of cases, history of previous hypersensitivity reactions to non-steroidal anti-inflammatory drugs (NSAIDs) was detected. No meaningful differences were found between those patients with aspirin sensitivity and those with aspirin tolerance neither in mean pre-bronchodilator FEV1 nor in ACT scores pre- and post-treatment. To conclude, aspirin sensitivity was not found to have an association with an unfavorable response to conventional treatment in patients with uncontrolled asthma.