A Study to Investigate the Effects on Blood Pressure and Pulse Rate of a Single 100mg Dose of Sildenafil in Patients With Benign Prostatic Hyperplasia Who Are Being Treated With Doxazosin

Inclusion Criteria: * patients who weighed between 132 and 220 lb (60 and 100 kg) with benign prostatic hyperplasia (BPH) * a total PSA (prostate specific antigen) measurement of less than or equal to 10 µg/L within the 12 months prior to screening, were currently on doxazosin treatment for BPH, had been treated with doxazosin for a minimum of 2 months prior to first dose of study medication, and had received a minimum of 4 mg doxazosin once daily (from doxazosin supplied for this study) for 2 weeks prior to the first dose of study medication. Exclusion Criteria: * subjects with a recent history (i.e. within 6 months prior to screening) of stroke or myocardial infarction * subjects with severe cardiovascular disorders such as unstable angina or severe congestive heart failure * subjects with known hereditary degenerative retinal disorders, such as retinitis pigmentosa * subjects with a supine systolic/diastolic blood pressure of \>170/100 mmHg or \<100/60 mmHg either at screening or at the pre-dose measurements * subjects receiving any other antihypertensive therapy other than doxazosin in the 3 weeks prior to the first dose of study medication.