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Parietaria pollen is one of the most important outdoor allergenic sources in all the Mediterranean countries, with a large number of subjects showing a positive skin prick test to both P. judaica and P. officinalis pollen extracts. A cross-reactivity between the two species has been already reported although few data are known at the molecular level. Twenty-five consecutive patients with Parietaria pollen allergy were selected on the basis of their clinical history. Skin prick test to P. judaica and officinalis extracts was performed. In vitro IgE measurement to both allergenic sources was performed by using a quantitative assay. ELISA inhibition experiments were made by using the rParj1 and rParj2 allergens. All the patients showed a positive skin prick test to both Parietaria species. Quantitative IgE measurement showed similar antibody concentration for P. judaica and P. officinalis extracts. ELISA inhibition experiments demonstrated that the cross-reactivity between the two species was due to the presence of the Parj1 and Parj2 allergens in the extracts with a high conserved IgE epitopes content. We conclude that P. judaica and P. officinalis pollens contain highly cross-reactive species-specific major allergens useful for the diagnosis and therapy of both allergenic sources.