Percutaneous skin testing to determine allergies is usually read 15-20 minutes after placement, but the time to reading may be prolonged because of clinic duties or emergencies. The objective of the study was to compare skin-prick testing (SPT) wheal and flare reactions at 10, 30, and 40 minutes with the standard 20 minutes to determine if extended time from placement to reading skin tests interferes with clinical significance. Fifty-three subjects undergoing routine aeroallergen SPT for allergy symptoms were tested with allergen extracts, histamine, and a negative control solution. Based on these results, SPTs can be read up to 40 minutes after placement but are more reliable when read between 20 and 30 minutes after placement. Skin testing to determine IgE-mediated or immediate hypersensitivity can be read up to 30 minutes without significant loss of reliability.