In recent years, we have become aware of a relatively high incidence of ohotoallergic dermatitis due to ingested and externally applied drugs at our OPD. These skin diseases have called our attention to the investigation on their basic mechanisms. These drugs are sulfonamides, benzothiadiazines, sulforylurea hypoglycemics, antibiotics, phenothiazines, and sodium cyclohexylsuilfamate, which is nowadays used very popularly as an artificial sweetening agent.