To examine the early use of phosphodiesterase-5 inhibitor (PDE-5i; sildenafil citrate) in preventing subsequent erectile dysfunction (ED) after (monotherapy) prostate brachytherapy (PB, an accepted option for Gleason 6 or low-volume Gleason 7 prostate cancer), as PB is currently being offered more frequently in younger patients, and ED can be a side-effect often within the first 12 months after treatment.