We have evaluated the predictive value of pelvic floor dysfunction (as characterized by abnormal contraction during defaecation straining or absent balloon expulsion) and evacuation proctography on symptomatic severe idiopathic constipation after colectomy with ileorectal or colorectal anastomoses for improvement and the use of laxatives. We also determined whether there was a relationship between the age of onset of symptoms and the symptomatic outcome after surgery. Forty-four women (mean age 34 years) were studied. Twenty patients had had a preoperative evacuation proctogram. Of these, 8 evacuated completely and 12 incompletely. Of the 29 patients in whom puborectalis electromyography was performed, 19 had paradoxal contraction on straining. There was no statistical relationship between symptomatic outcome and complete or incomplete evacuation on proctography, the presence or absence of puborectalis paradox or the age of onset of constipation (before or after the age of 10 years). Twenty-five patients had a preoperative balloon expulsion test. Of these, 14 were not able to expel a 50 ml water-filled balloon, and all 14 (100%) still experienced postoperative pain; 8 (57%) were still using laxatives. Of the 11 patients who were able to expel a balloon, 6 (55%) experienced pain and 1 (11%) still required laxatives postoperatively. The differences in postoperative pain and laxative requirements between those unable and those able to expel the balloon were statistically significant. Thus the balloon expulsion test may have predictive value when considering colectomy in patients with severe idiopathic constipation.