a) Signed Informed Consent b) Be adult males over 18 years of age diagnosed with erectile dysfunction and whose ED is attributable to an underlying, stable medical condition such as hypertension and atherosclerosis, antihypertensive medication, type I and type II diabetes mellitus, pelvic surgery and pelvic radiation, cerebrovascular accidents (stroke), multiple sclerosis, and Parkinson's disease; c) Participants must have been unable to have successful sexual intercourse for 3 months prior to study entry without specific ED therapy such as Vacuum Erection device (VED), ViagraTM (sildenafil), Cialis TM (tadalafil), MuseTM (alprostadil), or intracavernous injection therapy with an erectile function domain score of IIEF<21 at screening and baseline; d) Have been unable to tolerate, do not wish to continue, or have had unsuccessful results with, prior therapy for ED, e.g., ViagraTM, intracavernous injection therapy, MuseTM, or VED; e) If diabetic, documentation of HgA1c less than or equal to 8.0% prior to enrollment; f) If receiving medication for hypertension, documentation of blood pressure has been stable on the same medication for at least 2 months prior to enrollment; g) Be heterosexual and in a stable, monogamous relationship of at least six months duration; h) Agree to attempt intercourse with their partner at least four times per month while participating in the study; i) Agree not to use other treatments for ED while participating in this study; k) Have screening laboratory values and ECG that are within the normal range. See exclusion criterion below. l) Have a normal physical examination of the penis; m) If participant had a radical prostatectomy a PSA <0.4 for at least one year documented by 2 measurements during the preceding year; n) Be literate, able to give written informed consent, and comply with all study procedures and requirements.