OBJECTIVE To analyze the trough cyclosporine concentration-dose ratio (CDR) and its relationship to some commonly available factors such as cyclosporine dosage, patient age, grade of obesity, posttransplant days, serum creatinine, serum bilirubin, and serum cholesterol by multiple linear regression. METHODS The study was performed on 866 samples from 90 transplant recipients (25 kidney, 25 heart, 17 bone marrow, 13 liver, 10 simultaneous pancreas-kidney). RESULTS The results show differences between transplants both in cyclosporine CDR variability (expressed by the coefficients of variation) and in the capability of those factors to explain this variability (expressed by the coefficient of determination). Coefficients of variation were 41% for the 866 samples (from 34% in heart to 55% in pancreas-kidney transplantation) and 28% for the 90 patients' CDR mean values (from 24% in heart to 32% in pancreas-kidney transplantation). All factors, except for the grade of obesity, were related to the cyclosporine CDR for all transplants as a whole. However, differences in the influence of each factor on each transplant were observed. The coefficient of determination based on significant factors was R2 = 0.25 for all samples (from 0.18 in pancreas-kidney to 0.52 in liver transplantation) and R2 = 0.53 for the patients' CDR means (from 0.39 in heart to 0.83 in kidney transplantation). CONCLUSIONS We have quantified the cyclosporine CDR, its variability, and its relationship with some commonly available factors and found significant differences between transplant types. The equations of regression obtained might improve trough cyclosporine CDR estimation as a first step in cyclosporine dosage adjustment in kidney and liver transplant recipients.