Prostate cancer patients undergoing 125 I brachytherapy were reviewed. A relationship between pretreatment risk factors including International Prostate Symptom (IPS) score, urinary flow studies, and posttreatment urinary morbidity was assessed.Pretreatment IPS scores and urinary flow studies on 207 patients were reviewed. Relationship between scores and acute urinary morbidity was evaluated.Median age, 64 years; median baseline IPS score, 9; median prostate volume, 36cc. Catheterization was required in 18% of patients. Baseline IPS score and peak flow rate (PFR) varied inversely, demonstrating that PFR for patients requiring a catheter was lower than that for those not requiring catheterization. Univariate regression showed that prostate volume, prior hormone therapy, and PFR were statistically predictive of postimplant urinary retention. For every one-unit increase in PFR, the odds of catheterization decreased by 6%. Multivariate analysis demonstrated that only PFR and prostate volume were predictive of postimplant urinary retention.Pretreatment IPS questionnaire and urinary flow studies assist in predicting risk of urinary morbidity and retention post- 125 I brachytherapy for prostate cancer.