Fifty patients receiving oral contrast medium the day of their CT examinations were studied to evaluate colonic opacification. Two groups were given small volumes (20 versus 30 ml) of nondilute diatrizoate meglumine/diatrizoate sodium (Gastrografin) the night prior to examination and were compared with groups receiving no contrast medium (control) or a large volume (600 ml) of dilute barium the night prior to examination. Frequency of total colonic opacification was as follows: 30 ml Gastrografin. 92%; 20 ml Gastrografin. 71%; dilute barium. 46%; and control. 9%; Significant artifact was infrequent in all groups. Administration of small volumes of nondilute Gastrografin is a well tolerated, inexpensive, and effective method for opacifying the colon for CT.