This article summarizes evidence supporting the conclusion that the spontaneous rises in pressure that occur in the overactive bladder, particularly in detrusor instability (DI), regardless of etiology, are myogenic. The evidence quoted has been obtained by several groups of investigators and includes electrophysiologic studies of detrusor myocytes, tension studies of strips of smooth muscle, in vivo experiments on animal models, and light and electron microscopic studies of the structure of the bladder wall. The results of these studies demonstrate changes in the properties, structure, and innervation of the detrusor. These changes are consistent with the hypothesis that partial denervation of the detrusor may be responsible for altering the properties of the smooth muscle, leading to increased excitability and increased ability of activity to spread between cells, resulting in coordinated myogenic contractions of the whole detrusor. It is suggested that alterations in the properties of the detrusor myocytes are a necessary prerequisite for the production of the unstable pressure rises seen in DI of any origin.