The study objective is to:

1. To evaluate the effect of sildenafil citrate versus placebo on the IIEF_EF Domain at the end of the double-blind phase
2. To assess the relationship between treatment with sildenafil citrate or placebo and responses to the Quality of Erection Questionnaire (QEQ) and Sexual Experience Questionnaire (SEX_Q).

Inclusion Criteria:

• Male subjects aged 18-65.
• Documented clinical diagnosis of erectile dysfunction confirmed by an IIEF-EF Domain score of less than or equal to 25. (If the subject scores greater than 25 on the IIEF-EF Domain, screening procedures should be terminated.)

Exclusion Criteria:

• Subjects who have been treated with more than 6 doses of sildenafil citrate or any other phosphodiesterase type 5 (PDE-5) inhibitor such as vardenafil or tadalafil for erectile dysfunction [Note: subjects should not have taken any PDE-5 inhibitors within 4 weeks prior to the date of screening].