Ciclosporin, produced by the fungus Tolypocladium inflatum GAMS is endowed with potent immunosuppressive properties. Ciclosporin interferes with nuclear factors of activated T-cells, specifically by preventing cytokine gene transcription, particularly interleukin-2, which induces maturation and proliferation of helper T-cells. Initially, a traditional formulation of Sandimmun(R) was used, but its oral bioavailability varies. Absorption of a new formulation, Neoral(R), is significantly better and more reproductible. Ciclosporin has become a major agent in the prevention and treatment of organ transplant rejection. It is also efficient in treating various immune-related diseases. In dermatology, its efficacy has been proven in numerous double-blind, placebo-controlled studies for severe psoriasis and atopic dermatitis. Treatment with ciclosporin is also beneficial for many dermatologic diseases. However, the effect of this drug has principally been studied in open trials and in small cohorts. Ciclosporin provokes serious adverse reactions especially nephrotoxicity and arterial hypertension. Treatment is occasionally stopped because of these complications. The other common side-effects include muco-cutaneous, neurological and gastro-intestinal symptoms. Increased risk for malignancy is reported with ciclosporin. However, in patients with cutaneous diseases, the incidence is low.