We evaluated 224 consecutive patients referred for severe constipation prospectively by strict criteria to determine their candidacy for subtotal colectomy. Eighteen patients had insufficient symptoms to warrant evaluation. Two hundred six patients had anorectal manometry and defecography, and 182 had colonic transit measurement. Forty-nine patients had normal or minimally abnormal studies. One-hundred twenty-nine patients had abnormalities such as outlet obstruction, mild colonic inertia, diffuse gut dysfunction, or combinations of factors not favoring subtotal colectomy. Twenty-eight patients had colonic inertia without outlet obstruction and with disabling symptoms; 19 of these patients underwent subtotal colectomy. Follow-up > or = 12 months was available in 14 patients from this group. Of these patients, 12 (86%) were clinically improved. Preoperative evaluation accurately predicted postoperative fecal incontinence and likely reduced postoperative constipation. Small-bowel obstruction occurred postoperatively in 4 patients (29%), and remains a major risk of subtotal colectomy even in carefully selected candidates.