Background: The use of allergen immunotherapy is one of the definitive treatments for allergic diseases due to the cellular and molecular changes developed in the immune system response. The aim of this study was to assess the efficacy of subcutaneous immunotherapy in patients with allergic diseases presenting to the allergy clinic of Persian Gulf Hospital in Bushehr, Iran. Materials and Methods: Patients with  respiratory allergic diseases were enrolled in this observational study. The patients who had a positive skin prick test underwent immunotherapy for one year and were randomly included in this study. To evaluate the efficacy of immunotherapy, two questionnaires were completed for all the patients before and one year after the start of immunotherapy, including the Total Nasal Symptom Score (TNSS) and Asthma Control Test (ACT). Results: A total of 878 patients (53.6% male and 46.4% female) entered the study, including 45.8% who had allergic rhinitis, 25.5% with asthma and 28.5% with both diseases. The mean TNSS was examined for 258 patients before and after immunotherapy and was 13.64 and 5.83 (P<0.001), respectively. Moreover, the mean ACT score before and after immunotherapy was examined for 186 patients and was 1.71 and 17.82 (P<0.001), respectively. Conclusion: The mean TNSS and ACT score before and after the treatment showed a significant improvement in the patients’ symptoms in both genders, and women showed a relatively better response to immunotherapy. To conclude, immunotherapy is effective in patients with asthma and allergic rhinitis.