To evaluate the medium-term outcome of gluteraldehyde cross-linked (GAX) collagen in the treatment of genuine stress incontinence in women. The study comprised 111 women (age range 33-90 years) with genuine stress incontinence who were treated with para-urethral collagen injections between 1990 and 1995. The patients were followed prospectively and their clinical outcome documented. Pre- and post-operative urodynamic data were examined to determine any prognostic indicators. The overall results at a minimum 2-year follow-up (mean 3.3) showed 25% of patients to be dry and a further 40% improved. Although there were significant changes in some urodynamic values, no predictive factors of success were identified. Previous surgery for stress incontinence did not influence the final outcome. Para-urethral collagen injection is a safe and relatively simple procedure with acceptable results at the medium-term follow-up. It could be offered as a primary or secondary procedure to women with genuine stress incontinence who are unable or unwilling to undergo surgical treatment. Objective urodynamic assessment revealed no factors of prognostic significance.