The authors present a series of 30 unstable bladders treated only by perineal muscular reeducation by contact. The best results are obtained in the group presenting with pelvic floor hypotonia and USI (urinary stress incontinence) (efficiency 71.5%), especially when the closure pressure is normal (100%). The urodynamic control and the ominous disappearance of the associated USI confirm a muscular role of the treatment rather than a psychogenic one, the stabilization of the bladder being achieved through the perineo-detrusor inhibitory reflex. Because of this specific and efficient treatment the attention is drawn to the pelvic muscular weakness in the genesis of detrusor instability which is then called "deficitory instability". The interest of a clinical classification of detrusor instability is discussed.