This case of urticaria caused by exposure to light is reported because of its minute resemblance to the urticaria that occurs as a result of allergy. It is not reported as a case of allergy because of doubt as to whether it is actually an example of altered reactivity in the stricter sense of the word. It is possible that it is a case of normal reactivity toward a substance not ordinarily produced in the skin under the influence of light. Examples of sensitiveness to light are well known in medical literature, especially in relation to the diseases hydroa vacciniforme, xeroderma pigmentosa, and pellagra. The reaction has been attributed by many observers (Pfeiffer,1Betz and others) to the presence in the blood or tissues of photodynamic substances, such as hematoporphyrin.2 It was discovered accidentally by Raab that many fluorescent substances, such as eosin, fluorescein and hematoporphyrin, are photodynamic;