In a randomized clinical trail to assess acceptability, yields, costs, and unwanted effects of screening procedures, 232 subjects (137 with family history of colorectal carcinoma or adenoma, 95 without) were offered either flexible sigmoidoscopy or colonoscopy. Subjects with polyps found on sigmoidoscopy were followed up by colonoscopy. The two procedures were similar in compliance (65%) and yield (19% adenoma, 15% hyperplastic polyps). Polyps of either type were more common in those with a family history (prevalence: 41% compared with 24% without family history, p = 0.04). Costs per procedure were 60% lower for sigmoidoscopy, but follow-up colonoscopy reduced this cost advantage to 20% per subject. The subjects found the preparation for sigmoidoscopy easier, but the procedure more uncomfortable and embarrassing, as colonoscopy was performed under sedation. In this hospital-based study, colonoscopy was as acceptable to subjects, and only slightly more costly than sigmoidoscopy. Advantages of sigmoidoscopy would be greater for use outside hospitals and with less intensive follow up.