Thirty patients with asthma and/or monosensitized allergic rhinitis caused by grass pollen whose ages ranged from 15 to 35 years were selected. Two groups were established at random: an active group and a placebo group, and a double-blind study was done on treatment with immunotherapy for a period of 3 continuous years, with initiation doses administered according to the rush immunotherapy technique. Grass-pollen allergen extract Alutard SQ and histamine as a placebo were used. The objective parameters of efficacy evaluated were end-point cutaneous tests, conjunctival provocation, bronchial provocation, and symptom/medication scores, as well as specific immunoglobulin determinations. The statistical evaluation of the results was significant for the differences existing between the initial and final time of the active group, and there were significant differences between the two groups for all of the parameters considered. We found no relationship between clinical improvement and the range of specific immunoglobulin E values. Regarding the safety of the treatment, systemic adverse effects were manifested only in the initial phase (rush immunotherapy), and were easily controlled by treatment. We conclude that the efficacy and safety of immunotherapy with grass pollen make it possible to consider this treatment fundamental in these patients.