This is a multicenter, randomized, double-blind, placebo-controlled study to evaluate the effects on vision of six months of daily dosing of 5 mg tadalafil or 50 mg sildenafil compared with placebo.

Inclusion Criteria:

• Healthy male subjects or males with mild erectile dysfunction (ED).
• Between the ages of 30 and 65 years, inclusive.

Exclusion Criteria:

• Phosphodiesterase type 5 (PDE5) inhibitor therapy (tadalafil, sildenafil, or vardenafil) within 6 weeks of the start of the study.
• Diagnosis of diabetes mellitus.
• Any chronic illness or medication that is a risk factor for eye disease or any medication that causes retinal toxicity or affects visual function.
• Certain chronic medical conditions including unstable angina pectoris, severe renal [kidney] insufficiency, clinically significant hepatobiliary [liver, bile duct] disease, cancer, and AIDS/HIV.
• A history of clinically significant chronic ophthalmologic disease or any significant visual abnormality identified at the start of the study.