Sublingual-swallow immunotherapy (SLIT) using high doses of standardized allergen extracts has been found to be effective in reducing allergic symptoms and medication needs. A double-blind, placebo-controlled study was carried out in a large number of patients to determine whether medication needs can be reduced by SLIT. Some 136 patients with grass-pollen rhinitis with or without mild asthma were studied. Patients received either placebo or SLIT with a standardized grass-pollen extract administered daily with increasing doses up to 300 IR (index of reactivity) from January to the end of July 1994. During the grass-pollen season, patients were instructed to use medications as required and to visit their doctors in case of asthma. Symptom-medications scores were assessed during the pollen season, and serum-specific IgG4 was measured before and at the end of SLIT. In the SLIT group, drug consumption dropped significantly throughout the pollen season (P<0.02). Moreover, at the peak of the pollen season, betamethasone consumption was significantly reduced in the SLIT group (P<0.02). Only one patient in the SLIT group had an asthma attack compared to eight patients in the placebo group (P<0.02). IgG4 levels increased significantly in the SLIT group (P<0.001) but without correlation with symptoms. Side-effects were comparable in both groups. This study indicates that SLIT in grass-pollen rhinitis is well tolerated, improves overall clinical symptoms, and reduces drug consumption and the need for oral corticosteroids.