Calcium dobesilate has shown to improve endothelial function. This proof-of-concept clinical trial was done to check whether it may improve erectile dysfunction in diabetic men. Male diabetic patients with a diagnosis of erectile dysfunction were randomized to receive either calcium dobesilate 1 g twice per day or placebo for 6 weeks. The International Index of Erectile Function (IIEF) was chosen as the primary efficacy measurement. Statistical procedures included a pre-scheduled adaptive interim analysis to recalculate sample size. Relevant, but not significant differences in the mean change from baseline in the primary end-point (IIEF questions 3, 4 and 7) favouring dobesilate with respect to placebo were observed. Such differences reached statistical significance in some secondary end-points, including IIEF global as well as the erectile function and intercourse satisfaction domains' scores. Some patients experienced an important placebo effect. Results suggest that dobesilate may be of help to treat diabetic erectile dysfunction. Co-administration with phosphodiesterase inhibitors warrants further investigation.