The primary aim of this study is to assess how effective PF-00592379 is in restoring penile erections in patients with erectile dysfunction. Patients will attend for 4 treatment visits where they will receive the following in random order; 2 separate doses of PF-00592379, sildenafil 100mg used as a positive control, or placebo (dummy medication). The effect on penile erections will be assessed in the clinic using penile plethysmography (RigiScan PlusTM technique).