We prospectively assessed patients' erectile function (EF) using the International Index of Erectile Function (IIEF) and a global assessment questionnaire (GAQ) following permanent I-125-brachytherapy for localized prostate cancer. Of 378 treated patients, 220 had a minimal 2-y follow-up and 131/220 were sexually active prior to brachytherapy, with an EF domain score of greater than or equal to11 ( study group). Patients were allowed sildenafil at any time of the study. The patients' mean EF score, without excluding patients who used sildenafil, dropped within 3 months after brachytherapy, recovered at the end of the first year and remained unchanged for at least up to 2 y after treatment regardless of the addition of neoadjuvant hormone therapy to I-125-brachytherapy. Analysis of the GAQ revealed that 80% of the patients were satisfied with their sexual function up to 3 y after treatment. Any detrimental effect of permanent brachytherapy with or without the addition of hormone therapy on EF is reversible, and recovery is expected at 1 y after treatment in most patients.