Cyclosporine A (CsA) is metabolized into a vast spectrum of metabolites. The potential immunosuppressive action of CsAs metabolites has been studied extensively in the early 1990s, with conflicting and inconclusive results. Since then, the pharmacological and clinical consensus guidelines recommend the use of specific monoclonal assays for measurement of CsAs concentrations thus avoiding metabolite interference. Nevertheless, clinical benefit or superiority of these assays was never convincingly demonstrated. We provide a review of the performed in vivo, in vitro and animal studies and their conclusions. During the last years, many transplantation centres have employed the C2 monitoring of CsA (2 hours post-dose concentration). The metabolites / parent drug ratio two hours post dose is completely different from trough levels (predose concentration). Cyclosporine exerts its immunosuppressive action by inhibiting the enzyme calcineurin phosphatase (CaN). Currently, our laboratory, among others, is working on determination of the enzymes inhibition and its potential use as a pharmacodynamic biomarker. Utilizing this novel pharmacodynamic approach, we have performed in vitro and in vivo studies investigating the immunosuppressive impact of CsAs metabolites on C2 monitoring and on various monitoring assays (mono-/polyclonal). Interestingly, even though we have estimated in vivo that the potential immunosuppressive action of metabolites is less than 10% of the parent drug, we have found assays that take metabolites into consideration to correlate stronger with calcineurin phosphatase inhibition. We believe that the old controversial issue of metabolite induced immunosuppressive action examined under the light of newer pharmacodynamic approaches is still intriguing. Instead of a priori neglecting CsAs metabolites maybe we should investigate the potential of utilizing them as an additional tool towards better therapeutic drug monitoring of cyclosporine.