Anorectal and urodynamic studies were carried out in 10 young women with severe constipation and the results compared with those obtained in controls. The lowest volumes that provoked a desire to defecate (constipated 200 +/- 50 v controls 110 +/- 10 [mean +/- SEM] ml: p less than 0.05), and a desire to micturate (constipated 560 +/- 40 v controls 295 +/- 15 [mean +/- SEM] ml: p less than 0.001), were significantly greater in constipated patients compared with controls. The maximum tolerable rectal volume (380 +/- 30 v 290 +/- 20 [mean +/- SEM] ml: p less than 0.05) and the bladder capacity (720 +/- 50 v 540 +/- 10 [mean +/- SEM] ml: p less than 0.001) were also increased in the constipated subjects compared with controls. Electromyographic studies show failure of relaxation of the external anal sphincter (EAS) on attempted defecation in all 10 patients; and eight of these patients actually contracted their EAS when they strained to defecate, causing a functional outlet obstruction. Urodynamic studies showed normal urinary flow rates, normal detrusor pressures and normal radiology during voiding. Thus, these studies suggest that constipated patients have an increase in capacity and a reduction in sensitivity in the urinary bladder as well as in the rectum, but showed no evidence of obstruction to urine flow.