OBJECTIVE: To measure the prevalence of untreated hypertension in Australian adults, and examine the associations with clinical and lifestyle factors. DESIGN: AusDiab, a cross-sectional survey conducted between May 1999 and December 2000, involved participants from 42 randomly selected census districts throughout Australia. PARTICIPANTS: Of 20 347 eligible people aged >or= 25 years who completed a household interview, 11 247 attended a physical examination (response rate, 55%). MAIN OUTCOME MEASURES: The prevalence of hypertension (blood pressure >or= 140/90 mmHg or self-reported use of antihypertensive drugs) and its treatment; associations of clinical and lifestyle factors with the treatment of hypertension; and adequacy of treatment for primary and secondary prevention of cardiovascular disease. RESULTS: The prevalence of hypertension was 28.6 per 100 (95% CI, 25.0-32.3), and the prevalence of untreated hypertension was 15.2 per 100 (95% CI, 13.2-17.2). Of those with untreated hypertension, 80.8% (95% CI, 74.7%-85.0%) had had a blood pressure check within the preceding 12 months. At least one modifiable lifestyle factor was present in 71.7% (95% CI, 68.5%-74.8%) of participants with untreated hypertension. Although lower risk clinical characteristics of younger age and lack of hyperlipidaemia were independently associated with untreated hypertension, 53.5% warranted treatment based on current cardiovascular disease prevention guidelines and multivariable absolute risk assessment. CONCLUSIONS: Considerable scope remains for reducing the burden of cardiovascular disease through lifestyle modification and rational treatment of hypertension.