Background.To determine the impact of cyclosporine plasma protein binding on organ rejection after cardiac transplantation, the incidence of cardiac rejection episodes was compared among patients who had differing levels of cyclosporine plasma fraction unbound (fU).Methods.Forty-six consecutive cardiac transplant recipients were sampled at 1, 3, 6, and 12 months after transplantation, and cyclosporine plasma fUwas determined, using a specially developed equilibrium dialysis method. At the completion of the study, incidences of cardiac rejection episodes were compared among patients having mean cyclosporine fU(Csfu) that were low (ICsfu; mean ± SD, 1.33±0.10%, n=15), intermediate (ICsfu; 1.60±0.07%, n=16), and high (Csfu; 1.99±0.30%, n=15).Results.Percentage of endomyocardial biopsies (grade 3a, 3b, and 4) with respect to the total number of biopsies performed in the first 3 months after transplant was significantly higher in the LCsfugroup than the other groups (40.9% in LCsfuvs. 28.5 for ICsfuand 32.1% for HCsfugroups,P=0.02). The linearized rate of rejection (episodes of rejection/100 patient-days) in the first month after transplant was 6.5±1.7 for LCsfu, 3.5±0.8 for ICsfuand 4.3±0.9 for the HCsfugroup (P<0.05, low vs. intermediate-high). The mean (95% confidence interval) of time interval between the first and second episodes of rejections was 10.7 (5.6-16.0) days for LCsfu, 18.0 (8.6-29.0) days for the ICsfu, and 26.0 (15.1-36.9) days for the HCsfugroup (P<0.01). The total number of rejections requiring treatment per patient in the first 3 months after transplant was higher in the LCsfugroup compared with the others (4.0±1.7 episodes for LCsfuvs. 2.9±1.1 for ICsfuand 3.2±1.2 episodes for HCsfu;P<0.05). Four patients in the low group, one patient in the intermediate group, and no patients in the high group required treatment with total lymphoid irradiation (P<0.02).Conclusions.This finding suggests that patients with lower levels of cyclosporine fUare more prone to cardiac rejection and that the level of cyclosporine fraction unbound may be clinically important for determination of response to cyclosporine therapy.