Polysensitization is quite frequent in allergic children and may cause difficulties for the allergist in prescribing allergen-specific immunotherapy. This study aimed at evaluating the clinical effectiveness of 1 year of sublingual immunotherapy (SLIT) in a cohort of Italian allergic children with polysensitization. This open study was performed on 51 polysensitized children (34 boys; mean age, 11.8 years; range, 5.2-17.7 years) with allergic rhinitis and/or mild to moderate asthma. All of them were treated with SLIT for 1 year. The kind and the number of prescribed allergen extracts, the type of diagnosis, the severity of symptoms, and the use of drugs were evaluated at baseline and after 1 year. The adverse events to SLIT were also evaluated. Forty-two children were treated with a single extract, four with two different extracts and three with a mix of allergens. SLIT treatment induced a significant reduction in the number of sensitizations (p = 0.018); significant improvement of allergic rhinitis classification and severity; significant reduction of ocular, nasal, and bronchial symptoms (p < 0.01 for all); and drugs use (p < 0.01 for all drugs). No systemic reactions to SLIT were observed. This open study provides evidence that polysensitization is not an obstacle for prescribing SLIT in polysensitized children. Indeed, SLIT efficacy on clinical parameters is significant after 1 year and the therapy is safe.