Background: Allergen extracts have relatively short shelf lives, which limits their use and increase financial loss and waste on unused extracts. It is thus important to determine if efficacy persists beyond the expiration date. Objective: To determine the in vivo efficacy and bioavailability of outdated allergen extracts for diagnosis of allergic sensitizations. Methods: We enrolled 34 participants with allergic rhinitis and 5 participants with Hymenoptera hypersensitivity. After confirming allergen sensitization with the unexpired extracts, each participant had a second skin test with the matched outdated one (up to 7 years after the expiration date). All pairs of extracts were from the same company, stored under identical conditions, and tested for microbiologic contamination. The results of 356 skin-prick tests between expired and 111 unexpired extracts were compared. Results: None of the extracts had bacterial or fungal contamination. All outdated extracts produced a positive wheal reaction, with an average of 9.4 mm, which was not significantly different than the unexpired allergens. Seven years outdated lyophilized Hymenoptera extracts showed no significant differences in the wheal's size for the intradermal test at 1 μg/mL, between 5 and 9 mm. Conclusion: Outdated allergen extracts were safe and did not seem to differ in potency and bioavailability from unexpired extracts for the detection of allergen sensitization by skin-prick testing. These results supported our hypothesis that allergen extracts have efficacy and bioavailability that extend beyond the expiry date provided by the manufacturer. For the diagnosis of aeroallergens and Hymenoptera sensitization, it seemed that allergens can be used beyond the expiration date.