Background: There have been fewstudies conducted on the efficacy and safety of specific immunotherapy with allergen extracts of fungi compared with other allergen extracts, and there are no data on the major allergen Alt a 1 of the fungus Alternaria alternata. Objectives: We sought to evaluate the efficacy and safety of subcutaneous immunotherapy with 2 different doses of Alt a 1 in patients with rhinoconjunctivitis caused by sensitization to A alternata. Method: We performed a multicenter, randomized, doubleblind, placebo-controlled trial with Alt a 1 administered subcutaneously in patients with allergic rhinoconjunctivitis with or without controlled asthma aged 12 to 65 years. Three groups were included: the placebo group and active groups receiving 0.2 or 0.37 mg of Alt a 1 per dose. The main end point was the combined symptom and medication score. Secondary end points were cutaneous reactivity and serum IgE and IgG4 levels to Alta 1. Recorded adverse reactions were graded according to World Allergy Organization criteria. Results: There were significant reductions in the combined symptom and medication score for the 0.37-mg dose of Alt a 1 compared with placebo at 12 months of treatment. Reduced cutaneous reactivity and IgE levels, together with increased IgG4 levels, were demonstrated for the 2 active groups versus the placebo group. A similar safety profile was found for both active groups compared with the placebo group. No serious adverse drug reactions were reported. Conclusion: Immunotherapy with Alt a 1 was efficacious and safe, reducing the symptoms and medication consumption associated with rhinoconjunctivitis after only 1 year of treatment. The clinical benefits were associated with reduced skin reactivity and specific IgE levels and increased IgG4 levels.