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This article presents a new method for analyzing the spreading of skin erythemas. These occur as a result of the cutaneous vascular axon reflex which can be evoked by a noxious stimulation of the skin. Series of true-color images of the observed skin patch were recorded using a video camera. The images were digitized and stored on computer disk. The delineation of the reddening was segmented for every image of the sequence by a newly developed image processing method. Each image taken after the noxious stimulation was compared with the baseline before the stimulation and each image point was classified as: "unchanged" or "changed skin color." To improve the classification the CIE L *a *b * color space was used. The boundaries of the erythema were extracted from the resulting binary images. Every image of a sequence was analyzed in the same way in order to follow the time course of the flare response. The erythema reaction could be determined in an objective way using this methods. The automatically detected flare sizes were independent of human observers and had a high spatial and temporal resolution. It was used for a crossover study to assess the power of new drugs which modify the blood flow of the skin induced by an intradermal histamine application.