Four of 40 patients with resectable colon or rectal cancer had tumors causing acute large bowel obstruction with colonic dilatation; all 4 patients had preoperative CEA titers above 10 ng/ml with a mean of 28 ng/ml. Thirty-six cancer patients without acute colon obstruction had a mean CEA titer of 4.5 ng/ml; only 6 of 36 patients had circulating CEA titers 10 ng/ml or greater. This suggested that pre-treatment CEA titers in patients with obstructing cancer are unusually high. Multiple CEA assays were performed on two of the 4 patients with colonic obstruction before and after bowel decompressive procedures and prior to their definitive treatment. Relief of obstruction alone produces marked reduction in circulating CEA; this suggested that not only the extent of disease but also the pathophysiological changes associated with obstruction influenced circulating CEA levels.