Introduction: Erectile dysfunction (ED) is a common complication following prostate cancer treatment. Post-treatment erectile function (EF) preservation is strongly dependent on the baseline EF prior to treatment. Aim: To assess the baseline EF among patients with localized prostate cancer, and the factors associated with baseline EF. Methods: All men with clinically localized prostate cancer had their baseline EF assessed prior to brachytherapy at our institution. Six hundred ninety-nine men who completed the International Index of Erectile Function five-item questionnaires pre-treatment between 2001 and 2013 were included in the study. Data on patient factors (medical comorbidities and smoking history) and prostate cancer clinicopathological characteristics were recorded. Ordinal logistic regressions were used to estimate the effects of each variable on the severity of ED. Main Outcome Measures: Baseline EF among men with localized prostate cancer, and factors associated with ED. Results: Prior to permanent seed brachytherapy, 335 (48 ) patients reported no ED, 129 (17 ) mild ED, 42 (6 ) mild-moderate ED, 37 (5 ) moderate ED, and 165 (24 ) severe ED. In multivariate analyses, age, diabetes, and hypertension remained to be independently associated with ED, with diabetes most strongly associated with worse ED (odds ratio=2.6; 95 confidence interval=1.3-5.3). Conclusions: ED is common among patients with localized prostate cancer prior to any curative treatment. Assessment of baseline ED is important prior to curative treatment of prostate cancer in order to offer appropriate advise on likelihood of EF preservation post-treatment and avoid patient dissatisfaction with treatment outcomes due to unrealistic expectations.