Background: Radical prostatectomy (RP), external beam radiotherapy (EBRT), and brachytherapy are commonly utilized treatments for localized prostate cancer and may negatively impact sexual function to varying degrees. Patient-, disease-, and treatment-specific factors may all impact post-treatment sexual function. We aimed to evaluate predictors of post-treatment impotency, and develop a prognostic nomogram using prospective, patient-reported data from multiple validated health-related quality-of-life (HRQOL) instruments. Methods: Between 1999 and 2011, patient-reported data regarding treatment-related effects on erectile function were obtained from 2668 patients enrolled in one of four prospective longitudinal HRQOL protocols from the United States and Spain. Patients were treated with RP (n=1,294), EBRT (n=630), or brachytherapy (n=744). Although different HRQOL instruments were used in each protocol, questions pertaining to quality and frequency of erections were identical across the different instruments. Patient responses were obtained at baseline and 2 years after treatment. The endpoint of the model was impotency at 2 years post-treatment. Logistic regression analysis was used to model clinical information and outcome data. Internal validation was performed using bootstrapping. Results: 1,306 patients were potent at baseline and had 24-month follow-up. Differences in baseline patient characteristics such as patient age, ethnicity, and disease severity existed between the treatment groups. The impotency rate at 2 years was 62%, 53%, and 41% for patients treated by RP, EBRT, and brachytherapy, respectively. In multivariable analysis, age, PSA, modality of treatment, frequency of preoperative erections, diabetes, and hypertension were associated with post-treatment impotency (all p<0.05). A nomogram based on the predictive parameters had a concordance index of 0.726 and predictions were well-calibrated with observed outcome. Conclusions: A validated nomogram that predicts 2-year risk of impotency after treatment of localized prostate cancer has been developed and is anticipated to be useful for patient counseling regarding treatment options.