This study examined the frequency of medication errors in patients attending an outpatient transplant surgery clinic. Patients in the study were at high risk, taking an average of 11 medications daily. The investigators identified 149 errors in 93 patients, associated with a high risk of clinical adverse events such as hospitalization or rejection of the transplanted organ. Root cause analysis of the errors determined that the health care system was the cause of approximately one-third of the errors, and patient error (failure of the patient to accurately use the prescribed medication) accounted for two-thirds. Errors were identified at every point of the process, from the transplant team to the pharmacy to the patient, and the authors developed a classification system for errors incorporating each of these aspects. A prior study and commentary also discussed the contribution of patient error to the persistent problem of outpatient medication errors.