A prospective, randomized, 3-arm parallel trial on 45 males with ED that were never exposed to PDE5i therapy (naïve patients) will be enrolled.In each group, every patient will receive three treatment regimes (Viagra®50mg & Levitra®10mg, Viagra®100mg, Levitra®20mg), in different sequences of administration in such a manner that eventually each patient will receive all regimes in a double- blinded fasion.Safety will be evaluated at pre- screening by measuring hourly vital signs (blood pressure, heart rate)for 4 consecutive hours after taking half-dose combination. Any decrease in blood pressure of 20 mmhg below baseline will exclude the subject from the study. Effcacy will be evaluated by questionnaires (IIEF, Quality of erection questionnaire, grade of erection scale, Sear, QVS and Sexual Encounter Profiles for each sexual event). Non-parametric statistical analysis of the collected data Comparing the 3 groups will be performed.