Study the Safety and Effectiveness of Tadalafil in Men With Problems Getting or Maintaining an Erection When Taken Prior to Desiring an Erection

A Randomized, Double-Blind, Placebo-Controlled Study of the Efficacy and Safety of Tadalafil Administered "On Demand" to Patients With Erectile Dysfunction

Study to determine how safe and effective tadalafil is for Japanese men taking it when they need to get and keep an erection.

Inclusion Criteria:

• 3 months history of erectile dysfunction (ED)
• Anticipate a monogamous relationship with a female sexual partner
• Be able to make minimum required sexual intercourse attempts
• Abstain from using any other ED treatment

Exclusion Criteria:

• Other primary sexual disorders
• History of radical prostatectomy or other pelvic surgery that affected being able to have an erection
• History of penile implant or clinically significant penile deformity.
• Nitrate use
• Certain heart problems