To determine the risk factors for EDin men treated by prostate brachytherapy (PB) for localized prostate cancer and to propose a model to predict post-implant erectile function. Out of a series of 270 sexually active men treated by PB, 241 (89%) (mean age=66 years (range, 43-80)) accepted to participate in a mail-based study on erectile function. The risk factors for erectile dysfunction were determined by regression analysis and a predictive model was proposed. The performance of the model was determined in this population and subsequently verified in a population of 50 men treated by PB in another treatment center. The risk factors for ED after PB were age, the pre-implant IIEF score and prostate volume. In the studied population, the final model to predict a post-treatment IIEF-5 score, using these factors, had a sensitivity of 69% and a specificity of 68% associated to an area under the ROC curve (AUC) of 0.75. The same performance was obtained in another treatment center. Age, pre-implant IIEF-5 score and prostate volume may be used to predict post-implant erectile function in patients treated by PB.