Fourteen out of 18 patients (78%) had abnormal function 32 months after the modified Soave pull-through procedure for Hirschsprung's disease. The abnormalities were loose stools , perianal excoriation , soiling  and rectal prolapse . Excoriation was caused by stools containing an excess of water, particularly in the "extractable" phase, with raised Na+ and Cl- concentrations, osmolality and hydrogen ion (pH) but normal concentration of K+. A deficiency of dry stool solid was also demonstrated. Abnormalities were most marked in patients who had high inferior mesenteric artery ligation and residual proximal left colon. Faecal water abnormalities are due to failure of complete water and electrolyte re-absorption by the pulled-through bowel. Manometric studies indicate that post-Soave patients have a small, "spastic" rectum with poor reservoir capacity which leads to soiling. Perianal excoriation occurs when faecal composition is abnormal. Four patients were improved on small doses of oral Ispaghula husk fibre. Clinical improvements were associated with alterations in stool composition.