The invention relates to a transdermal therapeutic system, comprising a back layer that is impermeable to an active ingredient, a polymer matrix containing the active ingredient, and a protective layer that can be pulled off, wherein 5-aminolevulinic acid hydrochloride is used as the active ingredient and the basic polymer of the polymer matrix is an adhesive polyacrylate. Said transdermal therapeutic system is suitable for diagnosing and treating preliminary stages of skin cancer, such as actinic keratosis, and oncological skin diseases.
