This Protocol is a Randomized Prospective Study Comparing Prophylactic and on Demand Sildenafil Citrate Usage Administered During and After Radiotherapy With or Without Hormone Therapy Versus Radiotherapy With or Without Hormone Therapy Alone for the Preservation of Erectile Function After Therapy for Potent Patients With Clinically Localized Prostate Cancer

Inclusion Criteria:

- To receive external beam radiation therapy and/or brachytherapy for biopsy-proven
prostate cancer.

- Baseline International Index of Erectile Function (IIEF) erectile function domain
score >= 17.

- Patients receiving radiotherapy with hormone therapy undergoing a maximum of 9 months
of hormone therapy will be acceptable.

Exclusion Criteria:

- Baseline IIEF-EF (1-30) domain score of <17 pre-therapy

- Current routine use of erectogenic agents (use of agents > 4 times per month would
constitute "routine use")

- Neoadjuvant androgen deprivation therapy started more than 1 month prior to entry
into study (intake of Bicalutamide alone does not constitute exclusion from study
entry)

- Have a clinically significant penile deformity in the opinion of the investigator
(ie: Peyronie's Disease)

- Non-organ confined disease

- Prior prostate surgery or cryotherapy

- Prior prostate radiotherapy started more than 2 weeks prior to entry into study

- Currently taking 0.8mg Flomax daily

- Penile implant history

- Present at baseline with chronic angina requiring nitrates, angina occurring during
sexual intercourse or unstable angina within the last 6 months.

- History of myocardial infarction, coronary artery bypass graft surgery or
percutaneous coronary intervention within 90 days of baseline visit.

- History of significant cardiac conduction defect within 90 days of baseline visit.

- Exhibit systolic blood pressure > 170 or < 90 mm Hg or diastolic blood pressure > 100
or < 50 mm Hg at baseline visit or have a history of malignant hypertension

- Have any condition that would interfere with the subject's ability to provide
informed consent or comply with study instructions, would place subject at increased
risk, or might confound the interpretation of the study results

- Currently receive treatment with nitrates, cancer chemotherapy, or antiandrogens
(except finasteride taken as Propecia or Proscar)

- Have a history of drug, alcohol, or substance abuse within the past 6 months, as
assessed by the investigator.