Identifying barriers to hypertension management may facilitate cardiovascular risk reduction. Therefore, our objective, was to determine the prevalence of hypertension not managed with medication (‘untreated’) in a representative adult sample and identify patient factors/beliefs, and aspects of the patient-general practitioner (GP) relationship associated with untreated hypertension. The North West Adelaide Health Study, a biomedical cohort study over three stages from 2000-2009, assesses hypertension (systolic ≥ 140 mm Hg and/or ≥ 90 mm Hg or current treatment with anti-hypertensive medication), chronic disease and associated risk factors and health-care experiences, including risk perception, decision-making preferences, GP/primary care provider affiliation and satisfaction with care (n = 2425). The prevalence of hypertension was 32.1% (n = 781) comprised of treated (19.0%, n = 462) and untreated (13.1%, n = 319) hypertension. Thus, 40.8% of hypertension was untreated. Among hypertensive subjects, non-treatment was significantly associated with male sex, age < 45 years, workforce participation, infrequent GP visits, dissatisfaction with recent medical care, high total cholesterol, moderate-level physical activity and lower body weights. Compared with participants without hypertension (and no treatment), untreated subjects demonstrated significant (15%) 10-year Framingham general cardiovascular risk (odds ratio = 6.44, 95% confidence interval = 4.52-9.17). Novel screening strategies and public health messages to address beliefs and perceptions of both patients and the health system are required to identify untreated, at-risk hypertensive individuals.