The purpose of our study was to compare the two standard routes of urethral bulking injection in a prospective randomized trial. Forty women with genuine stress incontinence (n = 36), or mixed incontinence with a minor and controlled urge component (n = 4), participated in a urethral bulking agent trial. All patients had a standardized preoperative evaluation which included history, physical examination, assignment of incontinence status on a Stamey grading scale, postvoid residual (PVR) determination, Valsalva leak-point pressure, maximal urethral closure pressure, functional urethral length, Q-tip excursion angle, quantitative pad test, and completion of a quality of life questionnaire. On the day of injection they were randomly assigned to a periurethral or transurethral route of injection based on a computer-generated block randomization scheme. An ultrasound-determined PVR was obtained on all patients after injection. If self-catheterization was necessary, and the PVR was > 200 ml, urinary retention was diagnosed. Postoperative assessment included a patient interview, subjective assessment of improvement, PVR, voiding diary, and assignment of incontinence grade. At the screening visit there were no significant differences between the groups for any variables except type of stress incontinence. With short-term follow-up both transurethral and periurethral routes of injection seem to be equally efficacious. In the periurethral injection group there was a higher rate of postoperative retention; this group had a significantly higher volume of injectable agent used. There was no significant difference in risk of urinary tract infections between the two groups. We conclude that both periurethral and transurethral methods of bulking agent injection for stress urinary incontinence are equally efficacious, with minimal morbidity.