The aim of our study was to evaluate the effect of bacille Calmette-Guérin (BCG) therapy on erectile function in a cohort of male patients affected by non-muscle invasive bladder cancer. Thirty male patients undergoing BCG treatment for non-muscle invasive bladder cancer were enrolled in the study. Their mean age was 60.4 years. None of the patients had risk factors for erectile dysfunction (ED). All subjects underwent a BCG standard schedule therapy (once weekly instillation for 6 weeks). International Index of Erectile Function (IIEF-5) and International Prostate Symptom score (I-PSS) were addressed to the patients during the treatment schedule (at fourth or fifth instillation) and 1 month after the last instillation. The mean IIEF-5 score was 17.6 +/- 6.7 during therapy and 21.7 +/- 2.92 a month after the last instillation (P=0.008). Baseline ED and the association with lower urinary tract symptoms are variables significantly connected with post-treatment results (P=0.016 and 0.00 respectively) whereas the age seems not to be related to ED (P=0.256). No major side effects were recorded. It is concluded that BCG treatment is effective for prophylaxis of non-muscle invasive bladder cancer; however, it may induce a high incidence of ED. Although this effect is transient and reversible, erectile failure is another source of psychological distress that adversely affects the quality of life of men undergoing BCG treatment.