The Australian Diabetes, Obesity, and Lifestyle Study is a national, population-based examination of ∼11 000 adults with a third follow-up phase at 12 years. The aim was to use ambulatory blood pressure monitoring (ABPM) in a subsample (n = 508) of the main Australian Diabetes third follow-up cohort to determine the proportion with established, masked or white-coat hypertension in city and regional centers and the effectiveness of diagnosis and treatment.Mean age was 58.9 years, BMI was 27.6 kg/m with 53% women. The mean clinic BP was 127/73 mmHg and mean 24-h BP was 121/73 mmHg. Using regression analysis estimations, the predicted ABPM daytime equivalent for the hypertension threshold values of 140/90 mmHg were 136/90 mmHg. There were 43% classified as hypertensive due to either ABPM 24-h more than 130/80 mmHg (17%) or taking antihypertensive therapy (25%). Ambulatory SBP/DBP were higher in men (24-h + 6.4/4.9 mmHg, P < 0.001) compared with women. There was only 3% with white-coat but 21% with masked hypertension indicating 24% misdiagnosis. Based on ABPM, 9% were treated and still hypertensive, which was three times more common in men (14%) than women (4%). Thus 36% had not reached target. There were no differences between urban and rural participants. Based on ABPM, nearly half the participants were hypertensive while only a quarter were taking antihypertensive therapy.The findings highlight the importance of out-of-office BP assessments and the considerable gaps in effectively diagnosing and treating hypertension.