Detrusor instability is a major cause of urinary incontinence in females. Its cause is unknown. Diagnosis is made with cystometry. A phasic pressure rise during cystometry diagnoses the unstable detrusor, and a non-phasic pressure rise the stable "low-compliance" detrusor. The aim was to test the hypothesis that such cystometric findings may represent different levels of disturbance in peripheral feedback control of a prematurely activated, but otherwise normal micturition reflex. Of 169 neurologically normal female incontinent patients (mean age, 50; mean parity, 3) urodynamically tested, 40 had detrusor instability and 16 had low compliance. Digital support of bladder base tested the peripheral control mechanism, and hand-washing the central control mechanism. The data were applied to a non-linear feedback equation with one variable, X(NEXT) = cX(1 - X), where c = central inhibition and X = fraction of possible nerve impulses in the micturition circuit.