Background: Sublingual immunotherapy (SLIT) with monomeric allergoid, given according to the standard scheme, resulted effective and safe. However, the achievement of a clinical benefit requires a long time. We thus performed this study using an administration protocol starting in the co-seasonal period with a 3-day build-up phase and lasting only 6 months, in order to obtain the above benefit in a shorter time., Methods and Results: The study, prospective, randomised and controlled versus drug therapy, was conducted on 65 rhinitic and/or asthmatic patients allergic to Parietaria with or without other sensitisations. Twenty-four were allocated to 1,000 AU/week, 21 to 3,000 AU/week and 21 to drug therapy. They were treated from April to September 2006. At baseline, 3 and 6 months a Visual Analogue Scale (VAS) was performed to assess the patients' well-being. Drug consumption was evaluated by means of monthly diary cards. Bronchial reactivity was investigated at baseline and 6 months by methacholine challenge test. There was a greater VAS improvement in both the SLIT groups than in the controls after 6 months (p<0.05). In patients taking 3,000 AU/week this was already evident after 3 months. There was a significant reduction in rescue medication consumption between 3 and 6 months (p<0.05) in all three groups. The bronchial reactivity was reduced only in the SLIT groups (p<0.001). No adverse events were observed., Conclusions: At 6 months the allergoid SLIT showed itself to be effective and safe. In addition the subjective clinical benefit was obtained in a more rapid period, i.e. 3 instead of 6 months, when a higher maintenance dose was administered.