Although 1 million institutionalized elderly persons have urinary incontinence, little is known about the causes of this problem. We conducted clinical and physiologic studies to determine the causes of established incontinence in a representative sample of 605 institutionalized elderly persons (mean age, 89 years), of whom 40 percent were chronically incontinent of urine. Detailed urodynamic studies in 94 of the 245 incontinent patients (77 women and 17 men; 38 percent) showed that detrusor overactivity was the predominant cause in 61 percent, with concomitant impaired detrusor contractility present in half these patients. Other causes among women were stress incontinence (21 percent), underactive detrusor (8 percent), and outlet obstruction (4 percent). Among the relatively few men in this sample, outlet obstruction accounted for 29 percent of the cases. In 35 percent of the patients, at least two coexisting probable causes of incontinence were identified. Diagnoses among patients with impaired mobility or mentation differed little from those in unimpaired patients. We conclude that the pathophysiology of incontinence in this population is complex; that detrusor hyperreflexia with normal contractility ("uninhibited bladder") accounts for the minority of cases (29 percent), even among patients with dementia; and that the causes of incontinence are as diverse in severely impaired elderly persons as in those who are unimpaired.