Medication nonadherence after liver transplantation (LT) is associated with adverse clinical outcomes such as graft rejection and graft loss. Few studies have examined nonadherence and its impact on clinical outcomes in LT. The study objectives were (1) to evaluate medication understanding (with treatment knowledge and demonstrated regimen use scores) and medication adherence or nonadherence to entire regimens among LT recipients and (2) to examine associations of these exposures with clinical outcomes. We conducted a 2-site study of 105 recipients between 2011 and 2012 at 2 transplant centers in Chicago, IL and Atlanta, GA. Data were collected via detailed, in-person interviews and medical record reviews. Study participants were middle-aged and predominantly male; 15% of the sample had limited literacy. On average, patients were taking 11 medications [standard deviation (SD) = 4], and 39% had undergone a medication change within the last month. The average scores for the entire medication regimen were 86% (SD = 22%) for treatment knowledge and 78% (SD = 22%) for demonstrated regimen use. The mean score for self-reported nonadherence to the entire regimen was 14% (SD = 20%), whereas 32% of the patients were nonadherent according to tacrolimus levels. In multivariate analyses, lower income, less time since transplantation, a higher number of medications, and limited literacy were inversely associated with treatment knowledge scores (all P < 0.05), whereas limited literacy was associated with nonadherence according to tacrolimus levels (P < 0.05). In multivariate models, higher scores for treatment knowledge [incidence rate ratio (IRR) = 0.85, 95% confidence interval (CI) = 0.74-0.97] and demonstrated regimen use (IRR = 0.87, 95% confidence interval = 0.77-0.98) were independently associated with 15% and 13% reductions in the number of posttransplant rehospitalizations, respectively. Inadequate treatment knowledge and improper regimen use may be significant determinants of unintentional nonadherence among LT recipients and are associated with adverse clinical outcomes. Liver Transpl 21:22-28, 2015. © 2014 AASLD.