Background: Allergen immunotherapy (AIT) is the unique causal treatment for respiratory allergy. As AIT is expensive and of long duration, the availability of a marker predicting AIT responders is of crucial relevance. Objective: To investigate clinical parameters correlated with effective AIT in allergic rhinitis (AR) patients. Methods: This is a prospective, nonrandomized open study in which a total of 284 AR patients who had received house dust mite (HDM) subcutaneous AIT were enrolled from January 2011 to December 2015, and then followed up for 3 consecutive years. Demographic data, clinical history, laboratory tests (specific and total IgE levels), symptoms score, concomitant medication, and adverse reactions during AIT were collected. An AIT responder patient was defined when a visual analog score (assessing global symptoms) had decreased by >30% compared to baseline and concomitant medication was equal to or less than before AIT. Results: Thirty-three patients dropped out, so 251 patients were analyzed; 175 (69.7%) patients were responders. This group had a higher baseline symptom score than the AIT nonresponder group (7.5 vs. 6.9). A significant negative correlation was found between AR symptom duration and the clinical response to AIT. Local reactions (LRs) during AIT had a positive correlation. Other variables such as a family history of atopy, combined asthma history, and the levels of specific and total IgE had no correlations with effective AIT. Conclusion: Early intervention with AIT helps to improve the efficacy of AR treatment. LRs might predict successful AIT. Highly symptomatic AR patients may develop increased clinical responses to AIT.