The study objective is to evaluate the effect that sildenafil citrate has on the hardness of erections in males with erectile dysfunction, based on subject responses to question 5 on the Event Log (hardness of erection) as measured at the end of double blind treatment (week 6).

Inclusion Criteria:

• Male subjects aged 18-55
• 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]