A double-blind, placeo-controlled study was performed in order to confirm the safety, suitability, and efficacy of an alum-adsorbed Parietaria judaica-pollen allergoid, Allergovit®, for allergen-specific immunotherapy. Parietaria pollen is an important cause of pollinosis, particularly in the Mediterranean zone, where it may be encountered for up to 8–9 months of the year. It is an aggressive allergen, and the doses tolerated during immunotherapy are less than those achieved with grass pollen. This factor increases the desirability of using therapeutic preparations with minimal IgE-binding activity, such as allergoids, in order to reduce the risk of side-effects and enable patients to tolerate a higher dose of allergen, thereby increasing the chances of successful specific immunotherapy. Forty patients with rhinitis and/or asthma were allocated at random to active- or placebo-treatment groups at the beginning of the study. All patients received the active preparation during the second year of the study. Immunotherapy was well tolerated by all patients and the incidence of side-effects was low. Treatment resulted in significant reductions in specific cutaneous reactivity and increases in nasal tolerance. A progressive improvement in nasal inspiratory peak flow in association with the immunotherapy indicated a reduction in nasal inflammation. These objective assessments of efficacy endorsed the results from the patients’ diary cards, which indicated significant improvements in symptoms and reductions in the use of medication. The immunologic activity of the therapeutic preparation was demonstrated by the induction of a significant specific-IgG antibody response, with increases in IgG4 during the second year of treatment. We conclude that the safety and efficacy of immunotherapy with the Parietaria allergoid make it suitable for consideration in the treatment of patients with Parietaria-pollen-induced rhinitis or asthma.