In dogs, sphincter mechanism incompetence most often affects middle-aged spayed bitches and is more prevalent in docked animals, certain breeds and in dogs over 30 kg in weight. It results in urinary leakage, mainly when the animal is recumbent and relaxed. In humans, stress incontinence occurs most often in post-menopausal women who are multiparous, have had difficult deliveries and are obese. They also leak urine, but most often when active stressors increase abdominal pressure. Both women and bitches have shorter functional urethras and lower maximal urethral pressures. In women, the compliance and cross-sectional area of the urethra are increased and power generation is substantially weakened. Many women and most bitches have an intrapelvic bladder neck and a short urethra. Owing to hormonal deficiencies they have an impaired capacity to seal the lumen of the urethra and the urethral sphincter is less sensitive to sympathomimetics. In women partial denervation of the pudendal nerve is also a contributory factor.