Many patients with suspected colonic disease undergo rigid sigmoidoscopy, barium enema examination, and ultimately total colonoscopy, but the need for preliminary radiology has not been formally assessed. A total of 168 patients requiring large bowel investigation were therefore randomised to undergo either rigid sigmoidoscopy plus double contrast barium enema examination or total colonoscopy. Disease was found in 56 patients, including 14 with a carcinoma, 11 with polyps, and 16 with inflammatory bowel disease, the remainder having diverticular disease alone. Of the 89 patients allocated to double contrast barium enema examination, nine required a subsequent colonoscopy for suspected tumour or polyps, three because of incomplete radiological examination, and 12 for rectal bleeding for which no cause was found at the radiological examination. In 16 patients this yielded further information or altered treatment. Of the 79 patients undergoing total colonoscopy, only six required subsequent radiology. As both procedures were well tolerated with no major complications total colonoscopy may be the preferred initial investigation where facilities allow.