Publisher Summary Cyclosporin A is a neutral, cyclic undecapeptide derived from the soil fungus Tolypocladium inflatumgams. Cyclosporin A (CyA) has progressively gained a central role in the therapeutic approach of organ transplantation. Because of its high selective and reversible action on T lymphocytes, CyA is also used in autoimmune diseases, endogenous noninfectious uveitis, inflammatory bowel diseases, bronchial asthma, atopic dermatitis, and chronic urticaria. The chapter discusses the pharmacology, mechanisms of action and carcinogenicity of CyA. The association of CyA to carcinogenesis is mainly related to its immunosuppressive capacity. Severe immunosuppression leads to an increased incidence of malignancy, especially virus-related. The chapter describes the incidences of cancer after induction of CyA in cases of transplantation, autoimmune disease, rheumatoid arthritis, systemic sclerosis, lupus erythematosus, psoriasis, and few other immune-mediated diseases.