Antihypertensive medication treatment is one effective management strategy to prevent cardiovascular disease (CVD) and mortality. However, little research has been conducted on the rates of antihypertensive medication adherence and the effect of antihypertensive medication adherence on health outcomes in South Korea. We searched the Korean National Health Insurance Claims Database for records from 2003 to 2007. Patients in this study were 18 years of age or older and they were diagnosed with hypertension and newly prescribed antihypertensive medication in 2003. Adherence to antihypertensive medication was estimated as the medication possession ratio (MPR). Multivariate Cox regression was used to evaluate the association between medication adherence and adverse health outcomes after adjusting for patient demographics and clinical characteristics. Our study population consisted of 40,408 patients with a mean age of 51 years. Among the patients, 50.3% were men, 4.0% had Medicaid health insurance, 17.8% had diabetes, 20.9% had dyslipidemia and 42.4% were adherent (MPR ≥ 80%). Nonadherent patients (MPR<80%) were younger and more likely to have Medicaid health insurance; they had lower rates of diabetes and dyslipidemia compared with adherent patients. In the Cox multivariate analysis, nonadherence increased the risk of all adverse health outcomes, including all-cause mortality and hospitalization for CVD (hazard ratio: 1.57, confidence interval: 1.40-1.76). In conclusion, our study indicates that medication adherence is important for reducing hospitalization due to CVD and mortality.