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To detect causative allergens in type I hypersensitivity diseases, several methods have been used. Among those, skin prick test is easy to perform and sensitive. Several criteria presenting positive reactions have been used. Therefore, we tried to find out more sensitive and specific criteria for deciding a positive reaction on skin prick test in 57 patients(allergic rhinitis; 39, bronchial asthma ; 5, acute urticaria ; 2, chronic urticaria ; 11) with 69 allergens and compared the reproducibility between each positive criteria. We calculated the ratio of wheal size caused by each allergen to that of histamine and defined as "A" in case of it's ratio was over 0.5, "B" in case of over 0.7, "C" in case of over 1.0. The results were as follows, 1. The positive rate of house dust on skin prick test was the highest, and D. farinae, cat fur, D. pteronyssinus, mixture of cat fur, house dust, horse hair, dog hair in order. 2. Mean reproducibilities of four common allergens in each positive criteria were 91% in criteria A, 88% in criteria B, and 82% in criteria C. 3. Comparing between mean reproducibilities by earth positive criteria, in positive criteria between 0.5 and 0.7 and positive criteria between 0.7 and 1.0, there were no significant differences, but in positive criteria between 0.5 and 1.0, statistical significant difference was noted. In conclusion, it was suggested that the ratio of wheal sizes of allergen to histamine greater than 0.7 might be ideal for a positive skin prick test.