Ramadan can alter the course of diseases by changing nutrition patterns, sleep habits, and medication-taking schedules. There are some concerns that patients with asthma may be affected by these alterations during Ramadan and experience deterioration of their symptoms. This study aimed to investigate the effect of fasting in Ramadan on the severity of the disease and spirometric parameters in patients with asthma. An overall 120 patients with moderate to severe asthma were investigated during Ramadan and categorized into two groups of fasting (60 cases) and non-fasting (60 cases) groups. Patients underwent spirometry before and after Ramadan and asthma control status was also assessed. The parameters measured in spirometry were compared in each group before and after Ramadan and also between the two groups. Spirometric measurements including forced expiratory volume in the first second (FEV1), forced vital capacity (FVC), peak expiratory flow (PEF), and FEV1/FVC were not significantly different before and after Ramadan in both groups of fasting and non-fasting patients. Furthermore, there was no significant difference between the two groups in terms of these spirometric parameters changes from baseline. Nevertheless, FEV1 change in the fasting group was significantly higher than that in the non-fasting group (1.46±5.37 vs. -0.13±3.08, respectively; P=0.040). The results of this study demonstrated that fasting has no significant effect on the severity of asthma and spirometric findings in patients with moderate to severe asthma. Therefore, fasting during Ramadan can be considered safe for patients with asthma.