The significance of photodynamic substances in the process of photo-sensitization has frequently been demonstrated experimentally. Of these substances, the importance of hematoporphyrin in hydroa vacciniforme is conceded. In fact, this dermatosis is usually cited as the classic clinical example in which a photodynamic substance acts as the cause of photosensitization. We are not aware, however, that hematoporphyrin has been demonstrated in such superficial dermatoses as eczema or urticaria, although it is well known that these lesions appear as the result of hypersensitiveness to light. REPORT OF CASE A woman, aged 39, was first seen at the Mayo Clinic on Sept. 20, 1928. She gave a history of having had an eruption on the exposed parts of the body, especially the face and forearms, since early childhood. It was practically absent during the winter. She had repeatedly demonstrated to her own satisfaction that the eruption had a direct connection with exposure