Aim: To study the effects of homocysteine and copper on type 5 phosphodiesterase (PDE5) expression in cavernosal vascular smooth muscle cells (CVSMCs) and to investigate superoxide (O₂^·−) derived from nicotinamide adenine dinucleotide phosphate oxidase as homocysteine and copper generate O₂^·−, and O₂^·− upregulates PDE5 expression. Methods: CVSMCs derived from rabbit penis were incubated with homocysteine or copper chloride with or without superoxide dismutase (SOD), catalase, sildenafil citrate, or apocynin (nicotinamide adenine dinucleotide phosphate inhibitor) for 16 h. The expression of PDE5 and of glyceraldehyde‐3‐phosphate dehydrogenase (internal standard) was assessed using Western blot analysis. In parallel, O₂^·− was measured spectrophotometrically. Results: CuCl₂ alone (up to 10 μmol/L) and homocysteine alone (up to 100 μmol/L) had no effect on O₂^·− formation in CVSMCs compared to controls. In combination, however, homocysteine and CuCl₂ markedly increased O₂^·− formation, an effect blocked by SOD, catalase, apocynin, and sildenafil (1 μmol/L) when co‐incubated over the same time course. PDE5 expression was also significantly increased in CVSMCs incubated with homocysteine and CuCl₂, compared to controls. This effect was also negated by 16‐h co‐incubation with SOD, catalase, apocynin and sildenafil. Conclusion: This represents a novel pathogenic mechanism underlying ED, and indicates that the therapeutic actions of prolonged sildenafil use are mediated in part through inhibition of this pathway.