DERMATOLOGICAL therapy in the horse should be approached in the same way as it is in other species. Wherever possible, symptomatic therapy should be avoided and an accurate diagnosis made in order to facilitate the selection of an appropriate drug. Having selected the right drug, it is important that it is administered at the correct dosage and for an appropriate length of time. Many immune-mediated conditions require months of therapy, while others need lifelong immunosuppressive therapy. Similarly, deep infections need weeks or months of antimicrobial therapy, usually selected on the basis of culture and sensitivity testing. Such long-term treatment can only be justified when the disease process has been identified and a definitive diagnosis has been made. This article describes the use of systemic therapy for a range of infectious, allergic, immune-mediated and endocrine dermatological conditions. Topical therapy for these conditions will be discussed in Part 2 which is to be published in the next issue.