To investigate the effects of a single dose of sildenafil (100 mg) or placebo on blood pressure and pulse rate in subjects taking doxazosin for benign prostatic hyperplasia (BPH). To investigate the pharmacokinetics of doxazosin when co-administered with sildenafil 100 mg, and to investigate the safety and toleration of sildenafil 100 mg when co-administered 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.