Nonadherence to immunosuppressants may play a role in late rejection in liver transplant recipients. In children, emerging data suggest that adherence can be measured by computing the standard deviation (SD) of consecutive blood levels of tacrolimus, resulting in a number that reflects the degree of variability between individual measures (the Medication Level Variability Index, MLVI). A higher MLVI value means erratic immunosuppression, likely due to less adherence. Data on this method in adults are limited. We obtained data from the medical charts of 150 randomly selected adult recipients. The MLVI was significantly higher in patients who had biopsy-confirmed rejection (mean MLVI=3.8, SD=3.2) as compared with the rest of the cohort (mean MLVI=2.3, SD=1.5; p<0.01), and it was significantly higher in patients who had a rejection as compared with patients who had a biopsy that was not read as a rejection (mean MLVI=2.6, SD=1.6; p<0.01). The MLVI was both associated with rejection and predicted its occurrence. A threshold MLVI of 2.0 resulted in 77% sensitivity and 60% specificity in predicting rejection; a threshold of 1.8 resulted in a sensitivity of 92% and specificity of 48%. The Area Under the Curve (AUC) in a Receiver Operating Characteristic (ROC) curve analysis was 0.71 (95% CI: 0.61–0.81). The MLVI is associated with and can predict rejection, possibly related to nonadherence, in adult liver transplant recipients.