Purpose: The aim of this study was to investigate the efficacy of autologous platelet-rich plasma (PRP) in the treatment of stress urinary incontinence (SUI) due to intrinsic sphincter deficiency (ISD) refractory to medical treatment.Methods: Thirty-five patients with SUI due to urodynamically proven ISD were prospectively enrolled. Five milliliters of PRP (2.5–5 times the platelet concentration in peripheral blood) was injected into the external sphincter at 5 sites; all patients received 4 injections at monthly intervals. The primary end-point was the change in SUI severity as assessed by a visual analogue scale (VAS of SUI). The secondary-endpoints were the Global Response Assessment score and changes in urodynamic parameters from baseline to 3 months after treatment.Results: The mean age of patients was 68.7±12 years; the median duration of SUI was 4 years. Five patients had neurogenic SUI, while 30 had nonneurogenic SUI (21 with postprostatectomy incontinence, 6 with previous radical cystectomy, and 3 with other etiologies). Complete dryness was achieved in 7 patients (20.0%) while moderate improvement was observed in 14 (40.0%). The mean VAS of SUI score decreased significantly from 6.57±1.89 to 3.77±2.41 after treatment. The abdominal leak point pressure (ALPP) increased significantly from 98.3±55.8 to 157.3±79.3 cm H2O. There was no increase of ALPP in neurogenic SUI and less increase of ALPP in patients with failed treatment outcomes. No perioperative adverse events or severe complications occurred.Conclusions: Urethral PRP injection is safe and effective in increasing urethral resistance and improving SUI. PRP could be an alternative treatment modality for male and female patients with moderate SUI due to nonneurogenic causes.