Non-adherence to medications is associated with poor medical outcomes in adolescent transplant recipients. It is unclear whether non-adherence is further compromised when transplant recipients transition to the adult health care system. The purpose of the present study was to examine whether adherence changes during transition. We reviewed the medical records of 14 recently transitioned patients and compared their adherence and corresponding medical outcomes before and after transition. These outcomes were also compared with two cohorts of patients receiving care solely in pediatric or adult services. Medication adherence, measured through the use of standard deviations of tacrolimus blood levels, was examined for all patients. We found that adherence to tacrolimus significantly decreased after transition. After transitioning, patients furthermore exhibited poorer adherence than patients in the other two cohorts did over time. This small retrospective study suggests that the period of transition from pediatric to adult transplant clinics is a vulnerable one. Larger, prospective investigations of the transition process are necessary before recommendations are made regarding interventions.