: Although photosensitivity after photoallergy to topical phenothiazine antihistamines is well known, there have been no previous reports of dioxopromethazine inducing this phenomenon. : A housewife used 0.5% dioxopromethazine in Prothanon gel for palpebral pruritus and developed severe dermatitis of the lower eyelids with spread to the sun-exposed areas. : The minimal erythema doses and the minimal infiltrate doses for ultraviolet A (UVA) and ultraviolet B (UVB) were established before photopatch testing and at intervals up to 497 days thereafter. Test sites were read up to 144 hours after irradiation. Photopatch testing was performed with Prothanon gel, dioxopromethazine hydrochloride 0.001% to 0.5%, and the standard photopatch test tray (Hermal/Trolab). For patch testing, various series of the German Contact Dermatitis Group were applied. : Minimal erythema doses for UVA were diminished before photopatch testing and at intervals up to 500 days after Prothanon gel was discontinued. Exposure to UVB provoked abnormal delayed infiltrated reactions. Clinically the photosensitivity persisted within this period. Photoallergic reactions were seen with Prothanon gel, dioxopromethazine hydrochloride 0.005% to 1.0%, and promethazine hydrochloride 0.1%. The patient gave positive patch test reactions to various fragrance materials, balsam of Peru, costus oil, and propylene glycol. : Because topical dioxopromethazine may cause photoallergic contact dermatitis followed by long-lasting photosensitivity even after contact has been discontinued, its withdrawal from the market is recommended.