The purpose of the study is to investigate if regular treatment with sildenafil reduces blood pressure and improves blood vessel function in patients with hypertension (high blood pressure).
Inclusion Criteria: * Male or female * Hypertensive (office systolic BP ≥160 mmHg or diastolic BP ≥100 mmHg), not taking antihypertensive drugs OR hypertensive controlled (systolic BP \< 160 mmHg or diastolic BP \< 100 mmHg) on 1 antihypertensive agent * Hypertension confirmed on ambulatory monitoring (average daytime BP ≥145/95 mmHg) * Patients will also be included if they have 'borderline' hypertension and their calculated 10-year risk of cardiovascular disease is \>20% or they have target organ damage. Borderline hypertension will be defined according to ambulatory BP criteria alone (average awake systolic BP ≥135 and \<145 mmHg or diastolic BP ≥85 and \<95 mmHg). Exclusion Criteria: * History of other major cardiac, respiratory, neurological or renal disease * Systolic BP consistently \>210 mmHg or diastolic BP consistently \>120 mmHg * Systolic BP consistently \>180 mmHg or diastolic BP consistently \>110 mmHg in those withdrawn from existing therapy * Current alcohol abuse * Diabetes * Taking vasoactive drugs * Previous serious drug allergy * Pregnant * Participation, within 6 months, in other research studies