OBJECTIVES: Due to barriers such as cost, regimen complexity and perceived inefficacy, a substantial percentage of patients do not conform to medical instructions.  Onsite health centers with onsite pharmacies may improve medication adherence through convenient, holistic and relatively inexpensive care.  Accordingly, this study sought to evaluate the influence of these onsite pharmacies on medication adherence.  METHODS:   A retrospective analysis of electronic prescribing and claims data was performed to assess medication adherence among employees and their dependents that received medications from an onsite health center’s pharmacy compared with those that used an alternative site.  Specifically, the medication possession ratio (MPR) was evaluated for patients who received medication associated with treatment of asthma, depression, diabetes, hypertension or hyperlipidemia.  In addition, a subanalysis of MPR among condition management participants was performed.  RESULTS:   Overall, the MPR among patients who used the onsite health center’s pharmacy was higher than among those who used an alternate source: 54.8% versus 50.7%, respectively.  In particular, the MPR was significantly (P <0.002) greater for hypertension patients who used the onsite pharmacy compared with those who did not.  In general, the longitudinal analysis did not demonstrate significant differences between groups across time, indicating that medication adherence was relatively consistent over the study period.  Across conditions, medication adherence was significantly (P ≤ 0.001) higher among patients who participated in condition management programs than among those who were not enrolled.  CONCLUSIONS:   Both the onsite health center’s pharmacy and condition management program participation positively impacted medication adherence among employees and their dependents.