The objective of this study was to compare the efficacy of porcine dermal implant injection (Permacol) and silicone injection (Macroplastique) in the treatment of female urodynamic stress incontinence (USI) in a prospective randomized trial. Fifty women with urodynamically proven stress incontinence were recruited and randomised to receive either Permacol or Macroplastique injection. Twenty-five patients were enrolled in each case. An International Continence Society (ICS) standard 1-h pad test was carried out prior to the injection and a subjective analysis of incontinence made using a Stamey scoring system. In addition, a Kings College Hospital Quality of Health Questionnaire (KCQ) was completed. The women were followed up at 6 weeks and 6 months and the same methods used to gauge the success or failure of the operation. Preoperatively there were no significant differences in pad losses, Stamey score or King’s score between the two groups. The mean age of the women was 61 years (range 28–80 years). At 6 weeks there were significant reductions in the mean and median values in pad losses, Stamey score and King’s score in both Permacol and Macroplastique patients but the effects were more pronounced in Permacol patients than Macroplastique patients. Of the Permacol patients, 64% were improved on quantified pad losses out of which 60% were dry whereas 54% of Macroplastique patients were improved on pad losses of which 41.6% were dry. Of the Permacol patients, 64% and 60% had reduction in Stamey and KCQ score, respectively, whereas Macroplastique patients had 46% reduction in one or more grades of Stamey scores and 42% reduction in KCQ scores. At 6 months the results in the Permacol patients appeared to be sustained but not for Macroplastique patients. This study has shown that Permacol injection when used as a urethral bulking agent appears to have a higher cure rate for urodynamic stress incontinence than Macroplastique and these results persist until the follow-up period of 6 months. The use of Permacol injection is an attractive alternative in the treatment of urodynamic stress incontinence.