OBJECTIVES: Reducing hospital readmission helps contain healthcare cost and improve quality. The objective of this research is to explore the relationship between post-discharge medication adherence and hospital readmission. METHODS: A cohort study design was used to compare medication adherence rates among patients discharged from the hospital following a COPD (Chronic Obstructive Pulmonary Disease) or CHF (Congestive Heart Failure) related hospitalization.  Data from MarketScan® Commercial Claims and Encounters dataset (Thomson Reuters, New York, NY) for the period of January 1, 2008 to December 31, 2009 was used.  Inclusion criteria required that the patient be: (a) under age 64 (b) continuously enrolled over the pre and post 180 days of first (index) admission; (c) had a hospital admission during the first 6 months of 2009 for CHF (ICD9=428xx or 40211, N=12,260) or for COPD (ICD9 =490xx, 491xx, 492xx, 494xx, 496xx, N=149,411). Readmission defined as being readmitted within 30 days of discharge. The exposure of interest was a disease-related prescription claim following hospital discharge.  The primary outcome measured as a binary variable indicating hospital readmission within 30 days of initial discharge.  Multivariate logistic regression analysis was conducted controlling for previous hospital days (same disease), previous physician visits (same disease), co-morbidity, age, and gender. RESULTS: Patients lacking post-discharge medication claims had higher rate of readmission (p<0.01 for both disease). The differences were 17% (20% vs. 3%) for CHF and 3% (6% vs. 3%) for COPD patients. When controlling for confounders, CHF patients without claim were 8 times more likely to be re-admitted (p<0.01) than those with one. COPD patients showed similar effect with smaller effect size (Odds Ratio =3.8, p<0.01).  CONCLUSIONS: Patients without post-discharge medication claims are more likely to be readmitted. To improve the quality of inpatient care, efforts should be made to ensure patients adhere to medication regimes following hospital discharge.