Incremental infusion cystometry and continuous infusion urethrocystometry were performed on sequential days in 52 patients with an unstable detrusor. Bladder compliance was the same in both techniques. However, continuous infusion urethrocystometry demonstrated involuntary detrusor contractions in three times as many patients as incremental infusion cystometry. It is concluded that in the supine position, medium rate continuous infusion urethrocystometry is more discriminating than rapid rate incremental infusion cystometry with a Lewis recording cystometer in the demonstration of involuntary detrusor contractions.