To assess the early urinary morbidity experienced by the patients undergoing prostate brachytherapy by I125 permanent implants, and to identify the relevant clinical and dosimetric parameters. From May, 1998 and June, 2003, 600 patients were treated by the Institut Curie/Hopital Cochin/Hopital Necker group, by I125 transperineal interstitial permanent brachytherapy (TPIPB) for localized prostate cancer. All patients had no or moderate urinary symptoms (I-PSS < 15). In all cases, a systematic preventive treatment of the urinary symptoms was given (steroid or non steroid anti-inflammatory drugs and two different alpha-blockers). For the early urinary morbidity was analyzed at 2 and 6 months, and scored according to the RTOG classification. Multivariate analysis included clinical factors (age, T stage, PSA), hormonal therapy and its duration, ultrasound (US) dosimetric criteria (prostate volume, D90, D95, V100, rectal volume receiving more than 160 Gy), CT-scan criteria (prostate volume, D90, D95, V100, V150, rectal volume receiving more than 30 Gy, 50 Gy, 160 Gy and 215 Gy), and preventive treatments. Different cutoffs were used for each linear criteria. At 2 months, 19% of the patients were scored grade 0, 40% grade 1, 30% grade 2 and 9% grade 3, only 2% patients presented with a grade 4-toxicity (catheter). At 6 months, 54% of the patients were scored score 0, 36% grade 1, 8% grade 2, 2% grade 3 and 0% grade 4. Multivariate analysis shows that a total implanted activity >30 U (p < 0.0002), the first 200 patients (p < 0.002), a CT prostate volume >50 cc (p < 0.017) and a D90>180 Gy (p < 0.046) were independent factors for increased grade 2/3/4 urinary morbidity at 2 months. At 6 months, only a total implanted activity >40 U (p < 0.001) and T2 stage (p < 0.029) were found to be factors predicting for grade 2/3 urinary morbidity. Our results confirm that a significant urinary morbidity (RTOG ≥ 2) is observed in almost half of our patients at two months after TPIPB. The symptoms decrease thereafter; at 6 months only 10% of the patients reach a RTOG score of 2 or more.