Inclusion Criteria:

• Males with erectile dysfunction for more than six months according to the NIH Consensus Statement (the inability to achieve or maintain penile erection sufficient for satisfactory sexual performance).
• Stable, heterosexual relationship for more than six months.
• Documented written Informed Consent, from both the patient and his partner, after receiving adequate previous information and prior to any study specific procedures.
• An ED-EQoL score < or = 15.
• An IIEF score < or = 25.

Exclusion Criteria:

• Presence of penile anatomical abnormalities (eg. penile fibrosis or Peyronie's disease) that would significantly impair erectile function.
• History of radical prostatectomy.
• Retinitis pigmentosa.
• History of positive test for Hepatitis B surface antigen (HbsAg) or Hepatitis C.
• Unstable angina pectoris.
• History of myocardial infarction, stroke, electrocardiographic ischaemia (except stable angina), or life-threatening arrhythmia within the prior 6 months.
• Atrial tachyarrhythmia (eg. atrial fibrillation/flutter) with a heart rate of >100 beats per minute at screening.
• Child-Pugh class B liver disease or liver function abnormalities.
• Clinically significant chronic haematological disease or bleeding disorder.
• History of significant peptic ulcer disease within one year before Visit 1.
• Resting hypotension (a resting systolic blood pressure of <90 mm Hg) or hypertension (a resting systolic blood pressure >170 mm Hg or a resting diastolic blood pressure >110 mm Hg).
• Symptomatic postural hypotension within the six months of Visit 1.
• Uncontrolled diabetes mellitus (haemoglobin A1c > 12%).
• Patients who are taking nitrates or nitric oxide donors (e.g. molsidomine).
• Patients who are taking anticoagulants, with the exception of anti-platelet agents.
• Patients who are taking androgens (e.g. testosterone), trazodone or anti-androgens.
• Patients who are taking the following inhibitors of cytochrome P 450 CYP 3A4: potent HIV protease inhibitors (ritonavir, indinavir), the anti-mycotic agents itraconazole and ketoconazole (topical forms are allowed) or erythromycin.