The English and non-English language literature on allergen immunotherapy was reviewed for studies simultaneously using 2 or more distinct allergen extracts in either subcutaneous or sublingual immunotherapy. Thirteen studies were identified, 11 using subcutaneous injections, 1 using sublingual administration, and 1 using both. In studies with adequate information, administration of 2 extracts by means of either subcutaneous immunotherapy or sublingual immunotherapy was effective. In studies using multiple allergens, 3 studies showed clear efficacy, whereas in the other 2 studies, lack of efficacy might have been due to inadequate doses of extract or omission of clinically relevant allergens in the treatment regimen. It is concluded that simultaneous administration of more than 1 allergen extract is clinically effective. However, more studies are needed, particularly with more than 2 allergen extracts and with sublingual administration.