This is a study of the efficacy and safety of on-demand dosing of tadalafil or placebo administered for 12 weeks to men with erectile dysfunction caused by spinal cord injury.

Inclusion Criteria:

• males who developed erectile dysfunction as a result of spinal cord injury. Injury must have occurred 6 months before the start of the study.
• at least 4 sexual intercourse attempts during the first 4 weeks of the study without medication assistance
• no use of other erectile dysfunction treatment for at least 4 weeks before receiving the first dose of study drug. Also cannot use any other erectile dysfunction treatment during the study.

Exclusion Criteria:

• Impotence caused by other primary sexual disorders including premature ejaculation or untreated endocrine diseases such as diabetes, hypopituitarism, hypothyroidism, or hypogonadism
• History of radical prostatectomy or other pelvic surgery with subsequent failure to achieve any erection and history of penile implant.
• History of cardiac conditions including angina requiring certain treatment with nitrates; heart disease or coronary conditions including myocardial infarction, bypass surgery, angioplasty or stent placement for a specified time before starting the study.
• Significant renal problems within 6 months before starting the study. Also, active liver disease including jaundice at beginning of study.
• History of other central nervous system injuries, other than spinal cord injury, 6 months before starting the study.