The objective of this study was to estimate the prevalence and distribution of hypertension and to determine the level of awareness, treatment, and control of hypertension in the Portuguese adult population. The study was conducted in 2003, using a multistage cluster sampling method to select a representative national sample. A total of 5023 adults, aged 18 to 90 years, were examined. Three blood pressure measurements were obtained by trained observers using an OMRON M4-1 sphygmomanometer after a 5-minute seated rest. Information on history of hypertension and use of antihypertensive medication was obtained using a standard questionnaire. Hypertension was defined as mean systolic blood pressure > or =140 mmHg and/or diastolic blood pressure > or =90 mmHg, or use of antihypertensive medication. Overall, 42.1% of the Portuguese adult population aged 18 to 90 years, representing 3,311,830 people, would have hypertension. The age-specific prevalence of hypertension in the three age-groups studied--under 35 years, 35-64 years, and over 64 years--was 26.2%, 54.7% and 79% in men and 12.4%, 41.1% and 78.7% in women respectively. Among hypertensive subjects, only 46.1% were aware of their high blood pressure, 39.0% were taking antihypertensive medication, and 11.2% had their blood pressure controlled (<140/90 mmHg). Our results indicate that hypertension is highly prevalent in Portugal. The percentages of hypertensives who are aware of their condition, are being treated, and whose hypertension is controlled are unacceptably low. These results underscore the urgent need to develop national strategies to improve prevention, detection, and treatment of hypertension in Portugal.