Urodynamic studies were carried out on 30 patients with irritable bowel syndrome and 30 matched controls. Fifty per cent of the irritable bowel patients compared with only 13% of the control group had evidence of bladder dysfunction (p = 0.006). In the irritable bowel group detrusor instability was observed in 10 patients compared with only one control subject (p = 0.008). A steep cystometrogram occurred in five irritable bowel patients and three controls (NS). Detrusor instability was most common in patients with a bowel habit characterised by alternating constipation and diarrhoea. This is the first study to provide objective evidence that patients with irritable bowel syndrome may have a disorder of smooth muscle or its innervation that is not confined to the gastrointestinal system.