BACKGROUND AND OBJECTIVES: In this retrospective cohort study, we sought to elucidate the relationship between medication adherence (MA) and the incidence of complications in patients with newly diagnosed hypertension. SUBJECTS AND METHODS: Using claims data from the National Health Insurance Service, we measured health outcomes based on levels of MA, analyzed the incidence of complications in patients with a good MA, and clarified factors that may affect or predict MA. RESULTS: In 2008, a total of 4294773 patients were diagnosed with hypertension and were subsequently prescribed anti-hypertensive medications. In the present study, we enrolled 564782 patients who met our inclusion/exclusion criteria. The 40-59% medication possession ratio (MPR) group had a 1.36 times higher risk of developing complications (95% confidence interval [CI]: 1.27-1.45) than did the MPR≥ 80% group, as revealed through Cox's proportional hazards analysis. Similarly, the <20% MPR group was 2.01 times more likely to develop complications than the good MA group (95% CI: 1.82-2.23). Overall, patients who had a lower level of MA had a higher risk of developing complications. CONCLUSION: Our results demonstrate that MA is tightly correlated with hypertension health outcomes. Improving MA could be one strategy for reducing the risk of cerebrovascular disease complications and the loss of productivity in these patients.