Little is known about the causes of urinary incontinence in institutionalized elderly people, despite the fact that $8 billion is annually devoted to diapering those afflicted. We have identified a specific physiological abnormality--detrusor hyperactivity with impaired contractile function (DHIC)--that, although previously unrecognized, is the second most common (33%) cause of incontinence in this setting. Detrusor hyperactivity with impaired contractile function is a distinct physiological subset of detrusor hyperreflexia and presents with a seemingly paradoxical set of findings: the bladder is overactive but empties ineffectively. This imparied emptying is due to diminished detrusor contractile function and is associated with bladder trabeculation, a slow velocity of bladder contraction, little detrusor reserve power, and a significant amount of residual urine. Aside from its high prevalence, the importance of DHIC is that it may present as urinary retention, may closely mimic prostatic outlet obstruction, may explain why past therapeutic trials for detrusor hyperreflexia have failed, and may necessitate a change in the current nosology of bladder dysfunction. Furthermore, DHIC may represent a more advanced stage in the natural history of detrusor hyperreflexia, a stage characterized by deterioration of detrusor contractile efficiency. Thus, this previously unrecognized cause of incontinence in the elderly is common and raises several important issues.