Urethral stress relaxation was induced by sudden forced dilatation. The stress relaxation parameter, Pt0.5, was defined as the relative pressure decrease 0.5 s after dilatation, where the pressures at 0.1 s and at the new equilibrium were set to 100% and 0% respectively. Pt0.5 was calculated in 28 healthy and 30 women with genuine stress incontinence. Measurements were performed at the bladder neck, in the high pressure zone and in the distal urethra. The measurements showed that all along the urethra, Pt0.5 was statistically significantly greater in the healthy women than in the incontinent women, the greatest difference being found at the bladder neck. Although there was a slight overlap in values between the 2 groups, this parameter seems useful in differentiating between competent and incompetent urethral sphincters.