Summary. In a series of 368 incontinent women who presented to our urodynamic clinic for assessment, 232 (63%) were diagnosed as having genuine stress incontinence, and 136 (27%) as having detrusor instability. Obesity (>20% more than average weight for height and age) was significantly more common in women with genuine stress incontinence and detrusor instability than in the normal population. In those with detrusor instability the body mass index was found to increase with age and parity. In women with genuine stress incontinence the body mass index increased with age and the number of previous incontinence operations; it was higher in nulliparous than in parous women. There was no significant difference between obese and nonobese women in any of the urodynamic variables measured in the two incontinence groups.