Sublingual immunotherapy (SLIT) is increasingly used worldwide. Despite its safety being well ascertained, there is no universally accepted system to grade and classify its adverse events (AEs). According to the literature, it seems reasonable to classify and grade systemic side effects by using the previously published World Allergy Organization recommendations. On the other hand, local side effects are the most frequent with SLIT, sometimes leading to its discontinuation. Therefore grading of the severity of local side effects was perceived as necessary for the purpose of uniform reporting, classification, and quantification of this aspect. A World Allergy Organization Taskforce, after examining the available literature and the postmarketing surveillance data, proposed a clinically based grading of the severity of local AEs caused by SLIT. The use of the Medical Dictionary for Regulatory Activities nomenclature for AEs was also included in this context. The proposed grading system for SLIT-induced local reactions is expected to improve and harmonize surveillance and reporting of the safety of SLIT.