A meta-analysis of clinical trials of allergen immunotherapy was undertaken to assess the efficacy of this controversial form of therapy in asthma. A computerized bibliographic search revealed 20 randomized placebo controlled double-blind trials of allergen immunotherapy for asthma. The results extracted included asthmatic symptoms, medication requirements, lung function, and bronchial hyperreactivity (BHR). Categorical outcomes were expressed as odds ratios and continuous outcomes as effect sizes. The combined odds of symptomatic improvement from immunotherapy with any allergen were 3.2 (95% CI 2.2 to 4.9). The odds for reduction in medication after mite immunotherapy were 4.2 (95% CI 2.2 to 7.9). The combined odds for reduction in BHR were 6.8 (95% CI 3.8 to 12.0). The mean effect size for any allergen immunotherapy on all continuous outcomes was 0.71 (95% CI 0.43 to 1.00), which would correspond to a mean 7.1% predicted improvement in FEV1 from immunotherapy. Although the benefits of allergen immunotherapy could be overestimated because of unpublished negative studies, an additional 33 such studies would be necessary to overturn these results. Allergen immunotherapy is a treatment option in highly selected patients with extrinsic ("allergic") asthma.