Background: Sublingual-swallow immunotherapy (SLIT) is an accepted treatment for allergic rhinitis but its optimal dosage is scantly investigated. We studied the dose dependence of clinical efficacy and immunological response to SLIT by administering two different dosages of the same allergen in rhinitic children monosensitized to grass pollen. Methods: Seventy-one patients with comparable age and symptoms were randomized to receive SLIT by the same grass pollen extract from Stallergénes (Antony, France), 40 of them with the 100 IR and 31 with the 300 IR extract. All patients recorded diary cards for symptoms, medications and side-effects of the treatment, and had measurements of specific IgE and IgG4 in serum by the CAP System FEIA (Pharmacia, Uppsala, Sweden) and in nasal secretion by an in situ incubation method with the same reagents of CAP System FEIA. Results: Symptom/medication scores during the pollen season were significantly higher in patients treated with the lower dosage compared with those treated with the 300 IR dosage. Side-effects occurred with a comparable rate (25.8%vs 27.5%) in the two groups. Serum-specific IgE and IgG4 had no significant changes after 3 months of SLIT in both groups, while a significant seasonal increase of nasal IgE (P = 0.015) and IgG4 (P = 0.019) was found only in patients treated with the lower dosage. Conclusions: A rise of specific IgG4 and a blunting of seasonal increase of specific IgE in serum was repeatedly reported during subcutaneous immunotherapy (SCIT) with pollen extracts. Our findings show such blunting of specific nasal IgE along with a low symptom/medication score in patients treated with SLIT with the higher dosage, but not a concomitant rise of specific nasal IgG4. This suggests a local immunological effect of SLIT, different from systemic mechanisms of SCIT.