In transplantation, the advantage of therapeutical follow-up of immunosuppressive treatment involving cyclosporine is generally recognized, even though the ideal therapeutic index has as yet not been perfectly defined. Cyclosporine blood level determination is merely one factor among many others in therapeutical success, but if replaced in its context, can provide valuable and relevant information. A certain number of rules exist regarding the use, and pharmacological surveillance, of cyclosporine in all patients treated with this medicine. These rules take into consideration the patients immunological responsivity, the lenght of time since transplantation, the position of cyclosporine in immunosuppressive treatment, related pathologies and medicines. However, it is of special interest to consider a few points according to the nature of the indication. In kidney transplantation, and nephrotoxicity, determination of cyclosporine blood level will help differential diagnosis between an immunological origin (graft rejection) and an iatrogenic origin; in liver transplantation the consideration of metabolite determination, and study of metabolite ratio, will enable the gathering of information on performance status and toxicity hazards. In some cases, the necessity to administer intravenous cyclosporine, and the special weakness of some patients as in bone marrow transplantation, treatment surveillance patterns will be altered. Finally in spite of our lack of background information, the use of cyclosporine in autoimmune diseases has shown that principles of treatment and surveillance differed from one pathology to another, this being increased in some cases (juvenile diabetes), or occasional and even non-essential (psoriasis).