Polysensitization is very common in allergic patients and was previously reported to be associated with more severe symptoms and impaired quality of life. Polysensitization is often considered as a contraindication for specific immunotherapy (SIT). This study is aimed at evaluating the allergist attitude for decision making in choosing SIT in a cohort of Italian polysensitized patients. Moreover, the 1-year effectiveness of the prescribed sublingual immunotherapy (SLIT) in this cohort was evaluated. The study was performed on 244 patients (109 males, 135 females, mean age 28.7 years, S.D. 12.0) with allergic rhinitis (assessed by ARIA criteria) and/or mild to moderate asthma (assessed by GINA criteria) treated with SLIT for 1 year. The kind and the number of prescribed allergen extracts, type of diagnosis, severity of symptoms, use of drugs, and adverse events were evaluated at baseline and after 1 year. A total of 230 patients were treated with SLIT: 165 with a single extract, and 65 with two different extracts (mix). SLIT treatment significantly improved disease staging, and reduced symptom severity and drug use. No systemic reaction was reported. In conclusion, these findings provide preliminary evidence that SIT is effective and safe in polysensitized patients after 1 year of treatment also using single extracts, and thus does not represent an obstacle for prescribing SIT.