Measurements of bowel width and rectal area were used to define normal limits of size in 50 double-contrast barium enema films. Two types of patient with constipation were defined by comparison with these results. In 18 women with prolonged whole-gut transit time, the measurements were within the normal range. Of 11 men and 9 women with a previous radiologic diagnosis of idiopathic megacolon, all had an enlarged rectum and the abnormal width extended proximally to involve the colon for a variable distance. A width of 6.5 cm at the pelvic brim provides a convenient and discriminating separation of normal from abnormal.