Few studies have evaluated erectile function after interstitial radiation therapy for localized prostate cancer. Using a validated quality of life questionnaire, we assessed post-treatment erectile function and its relationship to treatment satisfaction and quality of life. We retrospectively reviewed the records of 171 consecutive patients who underwent Pd-103 or I-125 brachytherapy for prostate cancer between December 1992 and June 1998. Seventy percent of patients received neoadjuvant androgen deprivation therapy. All patients were mailed a validated questionnaire assessing sexual function and overall quality of life (UCLA Prostate Cancer Index and SF-36). Sixty-seven percent of all questionnaires were available for evaluation (114/171). The mean age was 69.1 y with a mean follow-up of 23 months (range 4-72, median 24). Seventy-one percent of patients (81/114) had pre-treatment erections sufficient for sustained vaginal penetration. Of these patients, potency was maintained in 49% of men (40/81). An additional 26% had erections firm enough for foreplay but not penetration (21/81). Erectile dysfunction rates were significantly lower in younger patients (48%) vs older patients (55%). There was no difference in post-treatment potency between men who received neoadjuvant hormonal therapy and those who did not (P>0.05). In addition, there were no differences in physical function (86, scale 0-100), general health perception (78), emotional well-being (83), energy/fatigue (74), and overall satisfaction (84) between men with erectile dysfunction and those without. In summary, two years following brachytherapy 25% of patients complained of complete (20/81) or partial (26%, 21/81) erectile dysfunction, for an overall rate of 51% (41/81). Short-term neoadjuvant hormonal therapy (<3-6 months) did not increase the likelihood of post-treatment erectile dysfunction. Interestingly, overall satisfaction rates among brachytherapy patients were high (84/100) and surprisingly did not correlate with post-treatment sexual function.