Calcineurin inhibitors, tacrolimus and cyclosporine, have been widely used to prevent the rejection or graft-versus-host disease after transplantations. Since these drugs have a narrow therapeutic range and show large inter- and intraindividual pharmacokinetic variability, frequent therapeutic drug monitoring is required to control their blood concentrations. Even with blood concentrations within the therapeutic range, some patients still experience acute rejection or infections. Tacrolimus and cyclosporine form a complex with their respective binding proteins, immunophilins, which in turn inhibit the phosphatase activity of calcineurin, a key enzyme in the activation of T lymphocytes. Pharmacodynamic assessment of calcineurin phosphatase activity in combination with the monitoring of blood concentrations has been studied. The inhibitory effects on calcineurin activity in peripheral blood mononuclear cells differed between tacrolimus and cyclosporine in transplant patients. The pharmacodynamics of both drugs shows great inter- as well as intraindividual variation, and acute rejection was associated with calcineurin activity. Calcineurin activity at trough time points was suggested as a single surrogate predictor for overall calcineurin activity throughout dosing periods. Monitoring of calcineurin phosphatase activity might be useful to determine the therapeutic range of tacrolimus and cyclosporine concentrations for an individual patient treated with a calcineurin inhibitor.