A group of 35 children affected by bronchial asthma, rhinitis and/or allergic conjunctivitis was examined; the children selected for the study had never undergone ITS before; all patients had received preventive drug and nondrug therapy for several months but none had succeeded in significantly reducing allergic clinical symptoms. Subcutaneous ITS Lofarma was prescribed for all patients (for various allergens) and blocking antibodies (IgG1 + IgG4) were assayed using a RAST system (Kit-Pharmacia) before the start of the study, and after 3, 9 and 12 months of therapy; 25 nonatopic children represented the control group. Although a certain number of allergen-specific blocking antibodies were present in all patients before the start of ITS, a significant increase in IgG1 + IgG4 specific allergens was observed after 9 months (especially for pollenosis). Compatible results were also obtained for blocking antibodies for children affected by dermatophagoides following 12 months of ITS. The aim of the present study was to evaluate the extent of the appearance of blocking antibodies in the circulation following ITS and to discover whether or not there was a correlation between their increased titre and improvement of the disease assessed by the use of clinical scores.