Background: New routes of administering immunotherapy in respiratory allergy are being studied as an alternative to conventional injective immunotherapy. We carried out a study to evaluate the clinical efficacy and effects of sublingual immunotherapy in patients with Parietaria judaica-induced respiratory allergy., Methods: A double-blind, placebo-controlled design was followed. Thirty patients with P. judaica rhinoconjunctivitis, mild asthma, or both were randomly chosen for sublingual immunotherapy (14 patients) or placebo treatment (16 patients). The patients underwent preseasonal rush induction treatment followed by coseasonal maintenance treatment during the Parietaria pollen season. Symptom and drug scores, as well as specific IgE and specific IgG4, were recorded., Results: Significantly lower symptom and drug scores were found (P=0.04), especially during the Parietaria pollination period, in the immunotherapy group. No significant difference in specific IgE and specific IgG4 was detected between the active and placebo groups; a statistically significant increase of specific IgE was detected in both groups (P=0.05). No patient undergoing active sublingual immunotherapy reported local or systemic side-effects., Conclusions: Our data suggest that sublingual immunotherapy is both clinically effective and safe in treating patients with Parietaria-induced rhinoconjunctivitis and mild asthma.