OBJECTIVES: Poor patient outcomes associated with low levels of adherence to medication have been previously reported in several chronic conditions. In transplantation, inadequate immunosuppression (IS) can lead to rejection and graft loss as well as increased healthcare costs. We evaluated patient and regimen characteristics associated with IS adherence in a private payer transplant population. METHODS: U.S. commercial claims data for de novo kidney, heart, liver, lung, pancreas and kidney-pancreas recipients transplanted 1995-2003 were linked to data from the Organ Procurement Transplant Network, a national transplant registry. All patients had =1 IS refill and were followed up to 1-year post-transplant. Patients who switched between study drugs were excluded. Medication adherence was measured by the medication possession ratio (MPR) i.e. ratio of number of therapy days supplied to number of follow-up days. The Kaplan-Meier product-limit method was used to estimate time to non-adherence. Adherence was defined as having a MPR = 90% and was modeled using multivariate logistic regression. RESULTS: In total, 349 transplant recipients met study criteria. Mean adherence to index IS therapy was 84% (MPR=0.84). Mean time to non-adherence was 8.4 months. Older age (OR=1.03, p=0.0005) and higher number of primary care visits (OR=1.02, p=0.03) were associated with higher odds of adherence. Hispanic ethnicity (OR=0.34, p=0.004), more comorbidities (OR=0.90, p=0.078), less than college education (OR=0.66, p=0.07), or a greater number of other prescription medications (OR=0.95, p=0.03) were associated with a lower likelihood of adherence. Copay and gender were not significantly associated with adherence. CONCLUSION: The mean level of adherence to immunosuppressive therapy was in congruence with previous findings for other chronic medications. Medication refill adherence was associated with age, race, comorbidity index, education status, primary care visits and number of medications taken. Drug copay did not impact adherence to immunosuppression therapy in this transplant population.